(CNN) As the calendar nears 2018, the battle for control of the Senate is shifting in all sorts of unexpected ways -- changes driven primarily by bad behavior but also a national political environment heavily colored by President Donald Trump's widespread unpopularity. 
 
 Consider this series of recent developments: 
 
 These three moments don't all work in one party's favor. A Jones win coupled with Bredesen's candidacy would clearly give Democrats momentum. The Franken resignation takes a seat that wouldn't even be on the map in 2018 -- his term isn't up until 2020 -- and makes it a problem that Democrats have to deal with in a year when they are already defending more seats than Republicans. 
 
 This series of events lands in what has already been a very unpredictable Senate election cycle. On its face, 2018 should be a banner year for Senate Republicans, with just nine seats to defend and 25 Democratic seats (not counting the now-open Minnesota seat) to target. Of those 23 Democratic seats, almost half -- 10 -- were carried by President Trump in 2016 including five (Indiana, Missouri, Montana, North Dakota, West Virginia) where Trump won by double digits. 
 
 It hasn't turned out that way -- yet! -- for Republicans as Trump's unpopularity has created a national political environment where Democratic candidates are coming out of the woodwork to run while top-tier Republicans are more hesitant to do so. 
 
 In Montana and North Dakota, two strongly Republican states represented by Democrats, Republicans have yet to land an "A"-type candidate. In Indiana and West Virginia, there are crowded primaries on the Republican side that will likely be nasty and expensive even before a dime is spent on the Democratic incumbents. 
 
 Arizona Sen. Jeff Flake into retirement and leading Nevada Sen. Dean Heller to And Steve Bannon-backed primary challengers are making life difficult for Republican incumbents, chasingSen. Jeff Flake into retirement and leadingSen. Dean Heller to brag that he "helped write" a tax bill that's widely unpopular. 
 
 Florida Gov. Rick Scott -- who Republicans hope will run against Democratic Sen. Bill Nelson next year -- put the heartburn Trump is causing even his staunchest allies in perspective when asked at a Republican Governors Association meeting last month whether Trump will help or hurt GOP candidates in the midterms. "We'll see what happens in 2018," Scott said. 
 
 Would he want Trump to campaign for him in Florida? "I don't know if I'm going to be a candidate. We'll worry about that next year," Scott said. Can Trump help Republicans on the ballot next year? "You'd have to ask them." 
 
 Not exactly a ringing endorsement. 
 
 The 2018 cycle started with Democrats expecting to play defense across the board. There were just too many seats to defend, and only two pickup opportunities (Arizona and Nevada) when they need to add three seats -- and hold onto all of their own -- to take control of the Senate. 
 
 If you asked Republicans at the start of the 2017 whether it was more likely that they would lose their majority or wind up with 60 seats at the end of 2018, they would have chosen the latter option every single time. 
 
 Now, the Senate majority looks in play -- even if Republicans still retain an edge due to the sheer number of Democratic seats in play and where those seats are. 
 
 If Jones wins next Tuesday in Alabama or Bredesen can turn Tennessee competitive -- and Trump stays unpopular -- the stakes get a lot higher. Add in Minnesota and the playing field is even bigger and less predictable. 
 
 It's not immediately clear whom that heightened unpredictability benefits. But Democrats surely never expected to be even sniffing at talk of regaining the Senate majority come 2019. And, for now, that's a possibility you wouldn't get laughed out of the room for bringing up. ||||| Breaking News Emails Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings. 
 
 / Updated By Jonathan Allen 
 
 WASHINGTON — Republicans may have just gotten another layer of armor to defend their Senate majority. 
 
 Sen. Al Franken's seat wasn't supposed to be up again until 2020. But his announcement Thursday that he was resigning amid allegations of sexual misconduct sets up a 2018 special election in Minnesota. And Republicans, who hold a 52-48 majority, are now on the hunt for a top-tier candidate in a politically competitive state where President Donald Trump lost by less than 2 percentage points last year. 
 
 The unexpected opportunity could be a "total game-changer in terms of control of the Senate," said Republican strategist Alex Conant, a Minnesota native who worked in communications roles for the Republican National Committee and former Minnesota Gov. Tim Pawlenty's 2012 presidential campaign. "If," he added, "we field a strong candidate." 
 
 Pawlenty, 57, is the dream candidate for the GOP, particularly after former Sen. Norm Coleman, who lost his seat to Franken by a few hundred votes in 2008, said Thursday that he won't run. Pawlenty currently serves as CEO of the Financial Services Roundtable, a Washington lobbying group, and is considered a good bet to raise a lot of money quickly. There was no word from him Thursday. 
 
 "Everybody in Minnesota on the Republican side is extremely eager to hear from Tim Pawlenty," said Josh Holmes, a Republican strategist and former chief of staff to Senate Majority Leader Mitch McConnell and who is from Minnesota. "He would be a spectacular candidate if he would consider it." 
 
 Holmes added that Franken's early departure "shifts the landscape" for control of the Senate. 
 
 The counterweight for would-be Republican hopefuls is the expectation that they would be running into a headwind in the first midterm of Trump's presidency. Typically, the president's party loses seats in Congress in midterms, and Trump's approval ratings have been mired around the 40 percent mark. Either way, a credible GOP candidate would force Democrats to defend a seat that wasn't even on the map until Franken's Democratic colleagues pushed him to resign. 
 
 In the interim, the state's governor, Mark Dayton, a Democrat, will appoint a successor to Franken who will serve until January 2019. But the winner of a 2018 special election, to be held alongside the other midterm Senate races, will finish the final two years of the six-year term Franken won in 2014. Dayton, who is reportedly considering naming Lt. Gov. Tina Smith as a caretaker, said Thursday that he's not yet ready to name an interim senator. 
 
 Tim Pawlenty speaks beside Michele Bachmann during the Republican presidential debate in Ames, Iowa, on August 11, 2011. POOL / Reuters 
 
 The Democratic bench in Minnesota is deep, as the party holds all of the partisan statewide elected offices, and Reps. Tim Walz and Keith Ellison have long been considered potential contenders for an open Senate seat. One senior Democratic Senate aide pointed to the state's Attorney General, Lori Swanson, as a potential candidate. And Jake Sullivan, who was Vice President Joe Biden's national security adviser and a top State Department and campaign aide to Hillary Clinton, has told friends in the past that he might seek office in Minnesota. 
 
 Ellison, who is the first African-American to represent Minnesota in Washington and the first Muslim elected to Congress, could be the most intriguing candidate to watch in a contested Democratic primary. A longtime leader among House progressives, he elevated his national profile as a surrogate for Bernie Sanders in 2016 and as the runner-up candidate for Democratic National Committee chairman after the campaign was over. 
 
 Jim Manley, a former Senate Democratic leadership aide and Minnesota native, said Democrats should be able to hold serve in Minnesota. 
 
 "There's no such things as free shots in politics, but I feel pretty confident that Democrats can keep the seat in 2018," Manley said. "I understand that Trump did very well there, but I think of the [Democrats] being discussed, there's a whole bunch of people who can win in 2018." 
 
 For Republicans, Pawlenty is in a class by himself as a potential candidate. But if he doesn't run, former Rep. Michele Bachmann and several members of the current Minnesota delegation in the House, including Erik Paulsen and Tom Emmer, could be attractive alternatives for the GOP. 
 
 Though Minnesota has long been in Democratic hands at the presidential level, having last voted for a Republican in 1972, Republicans have won seven Senate elections in that span. 
 
 "It’s certainly a purple state and one that with the right candidate Republicans could compete to win," Holmes said. ||||| Video (11:36) : From the Senate floor Sen. Al Franken announced his resignation but promised to fight alongside his constituency. 
 
 – Facing a barrage of sexual harassment complaints and calls to step down from friends and foes alike, Sen. Al Franken took to the floor of the U.S. Senate Thursday to announce he would resign — a swift and historic fall for an unlikely Minnesota politician who had become one of the Democratic Party’s most recognizable leaders. 
 
 Franken was quick to explain that he was stepping down not because he thought he had done something wrong, but because he had determined that Minnesotans deserved a senator who wasn’t distracted by mounting allegations and a looming Senate Ethics Committee investigation. 
 
 “Minnesotans deserve a senator who can focus with all her energy on addressing the challenges they face every day,” Franken said in a speech that lasted just over 10 minutes, as his wife and adult children looked down from the Senate gallery. 
 
 With Franken’s imminent exit, DFL Gov. Mark Dayton now faces a high-stakes decision about whom to appoint as replacement. One option is a temporary fill-in who would not run in a November 2018 special election. Top Democrats, particularly in Washington, favor a replacement who wants to run, since occupying the seat for nearly a year would offer a strong head start. 
 
 Even as Franken bowed to his political reality, saying he could no longer be effective, he sought to clear his name. 
 
 “I know there’s been a very different picture of me painted through the last few weeks, but I know who I really am,” Franken said. Of the claims against him by more than half a dozen women, he said: “Some of the allegations aren’t true. Others I remember differently.” 
 
 Video (03:11): Al Franken's rise and fall Video (03:11): Al Franken's rise and fall 
 
 Even as his political career crumbled, Franken sought to draw a distinction between himself and two Republicans also accused of mistreatment of women, President Donald Trump and Alabama Senate candidate Roy Moore. 
 
 “I of all people am aware that there is some irony in the fact that I am leaving while a man who has bragged on tape about his history of sexual assault sits in the Oval Office and a man who has repeatedly preyed on young girls campaigns for the Senate with the full support of his party,” Franken said. 
 
 As two dozen fellow Democrats and a single Republican colleague looked on, Franken delivered a defiant political eulogy for his eight years in the Senate. 
 
 “I know in my heart that nothing I have done as a senator — nothing — has brought dishonor on this institution,” Franken said. He later added: “What I want you to know is that even today, even on the worst day of my political life, I feel like it’s all been worth it.” 
 
 Reverberations back home 
 
 Franken’s departure will shift Minnesota’s political landscape for years. There will now be two U.S. Senate seats on the 2018 ballot, along with an open governor’s race. Five of Minnesota’s eight House seats could also be up for grabs, making for a political cycle that’s likely to see tens of millions of dollars or more flowing into the state. 
 
 Dayton is expected to appoint a replacement soon. A Democratic source told the Star Tribune that Dayton is likely to appoint Lt. Gov. Tina Smith, and that she is not expected to run in the special election. 
 
 But other ambitious DFLers are likely to be interested or at least considered. In a statement issued after Franken’s speech, Dayton said, “I expect to make and announce my decision in the next couple days.” 
 
 Franken was not specific about when his resignation would take effect, saying only it would be “in the coming weeks.” A spokesman later said no definite timeline had been set, and that Franken would continue to perform his duties until then. 
 
 Jennifer Duffy, senior editor at the Washington-based Cook Political Report, which provides nonpartisan political analysis, said Franken’s delay in leaving is generous to his staff members, who will likely be out of work when he resigns. It also gives Dayton more time to settle on a replacement. 
 
 She said Dayton may want to appoint a “caretaker,” like Smith, though Senate Democratic leaders would probably prefer a replacement who could immediately begin campaigning. 
 
 Dayton condemned Franken’s behavior while still praising him as “very smart, very hardworking and very committed to Minnesota.” 
 
 “I extend my deepest regrets to the women, who have had to endure their unwanted experiences with Senator Franken,” Dayton said. “As a personal friend, my heart also goes out to Al and his family during this difficult time.” 
 
 Allegations mounted 
 
 Franken’s resignation capped three excruciating weeks of revelations and accusations from women who said they’d been groped, kissed or embarrassed during encounters with him over the years. 
 
 It started with a former USO entertainer who shared a photo of a grinning Franken, mugging for the camera with his hands hovering over her breasts as she slept during a flight back from Afghanistan in 2006. She also said that Franken kissed her without her consent while on the tour, and that he “mashed his lips against mine.” 
 
 Gallery: Photo gallery: Al Franken through the years Gallery: Photo gallery: Al Franken through the years 
 
 Within days, a Minnesota state fairgoer told CNN that her photo-op with the newly elected Franken ended with his hand clamped around her buttock. 
 
 When a seventh accuser came forward Wednesday, describing a kiss she said Franken tried to plant on her after a 2006 radio show, Franken’s fellow Senate Democrats decided they’d had enough. Calls for his resignation peppered social media, starting with a half-dozen female Senate Democrats and snowballing until more than half the party’s caucus had called for his ouster. 
 
 Even before his colleagues called for him to resign, Franken’s stock in the Senate had plummeted — from a permanent fixture at party fundraisers and campaign rallies to a figure of public scorn, whose donations to other candidates were regifted to shelters for abused women. 
 
 Franken said Wednesday he still believed the Ethics Committee would have cleared him, and also said he feared his attempts to be respectful of the accounts of his accusers gave the “false impression” that he was admitting to the allegations. 
 
 “I think that was the right thing to do,” he said. “I also think it gave some people the false impression that I was admitting to doing things that in fact I haven’t done.” 
 
 Franken had issued a series of apologies in response to each allegation, because, he said, “all women deserve to be heard and their experiences taken seriously.” He did not repeat the apologies Thursday. 
 
 Some longtime supporters and allies continued to stand by him. 
 
 “Al has been an ardent advocate for women’s rights and the people of Minnesota. I am grateful to have had the opportunity to work for him, and always will be,” said Natalie Volin Lehr, who previously worked in Franken’s office as political director and women’s policy adviser. 
 
 Sen. Al Franken, D-Minn., center, held hands with his wife, Franni Bryson, as he left the Capitol after announcing that he will resign in the coming weeks. 
 
 “I am also a lawyer and firmly believe in due process — it is an absolute travesty that it has not been afforded to him.” 
 
 Difficult time 
 
 A mob of reporters and photographers staked out a Capitol entrance for hours Thursday morning, waiting for Franken to appear. When he appeared with wife, Franni, at his side, they forced their way through the throng as several people in the crowd shouted for them to be let through. 
 
 The senators who gathered on the floor Thursday included several former allies who demanded Franken’s resignation a day earlier. They listened in grim silence, a few wiping at their eyes. After his speech, they made themselves scarce. 
 
 “I’m not going to talk about Senator Franken but thank you very much,” Sen. Claire McCaskill said as a reporter approached her. The Missouri Democrat was among the first to call for Franken’s resignation. 
 
 Sen. Amy Klobuchar, Franken’s fellow Minnesota Democrat, said she privately urged him to resign when they spoke Wednesday. The next day, she was first to arrive in the Senate chamber. She spoke to the long line of Franken staffers who sat watching at the back of the chamber. 
 
 “I didn’t think that many of our colleagues would come, honestly, after calling for his resignation,” Klobuchar said. “I thought it was good that there were a number of people there.” 
 
 As for the speech itself, she said: “I know he didn’t really apologize to the people, and that would have been nice. But I think the bigger deal for me was that he was able to talk with a lot of love about our state, and what he liked about his job and what he wanted to be his legacy.” 
 
 Sen. Al Franken is accompanied by his wife, Franni Bryson, as he arrives Thursday morning at the Capitol in Washington, Dec. 7, 2017. Franken announced Thursday he would resign from the Senate after his support among Democrats crumbled, becoming the highest-profile casualty in the growing list of lawmakers felled by charges of sexual harassment or indiscretions. 
 
 That legacy looked like it would be very different a few weeks ago. After eight years in the Senate, Franken had emerged as a powerful voice for progressive causes and forceful critic of the Trump administration, getting national attention with aggressive questioning of Trump’s Cabinet officers. 
 
 Franken’s voice 
 
 As Franken stepped down, he went down swinging. He was giving up his Senate seat, he said, “but I am not giving up my voice.” 
 
 He said he would “continue to stand up for the things I believe in as a citizen and as an activist.” 
 
 Talking about the national debate shifting to focus on the mistreatment of women and consequences for their abusers, Franken said, “I was excited about the conversation and I hoped it would result in real change. Then the conversation turned to me.” 
 
 The same forces that pushed Franken out of office — women speaking out publicly against men who use their power to hurt and harass others — have pushed countless newcomers into politics for the first time this year. That’s good, Franken said, offering up his own painful fall as an object lesson. 
 
 “You too will experience setbacks and defeats and disappointments. There will be days when you will wonder whether it’s worth it,” he said. “For a decade now, every time I would get tired or discouraged or frustrated, I would think about the people I was doing this for and it would get me back up on my feet.” 
 
 After his speech, Franken and Franni left the Capitol, hand in hand, chased by a horde of reporters. 
 
 Sen. Al Franken went to the Senate floor Thursday to announce that he's resigning his seat. 
 
 Asked if he had anything to tell the people of Minnesota, Franken replied: “I’m coming home.” ||||| FILE - In this Aug. 29, 2012 file photo, former Minnesota Gov. Tim Pawlenty looks over the main stage during a sound check at the Republican National Convention in Tampa, Fla. U.S. Sen. Al Franken's announcement... (Associated Press) 
 
 FILE - In this Aug. 29, 2012 file photo, former Minnesota Gov. Tim Pawlenty looks over the main stage during a sound check at the Republican National Convention in Tampa, Fla. U.S. Sen. Al Franken's announcement Thursday, Dec. 7, 2017 that he will resign triggers a mad-dash special election in 2018... (Associated Press) 
 
 ST. PAUL, Minn. (AP) — Minnesota Sen. Al Franken's departure from the Senate solves one problem for Democrats, demonstrating their will to push out one of their own when sexual harassment allegations pile up. Now they must worry about keeping one more vulnerable seat when they already had their hands full in next year's election. 
 
 Franken said Thursday he would resign, just a day after new allegations brought the number of women alleging he had groped or tried to kiss them to at least eight. 
 
 In a sometimes emotional speech delivered on the Senate floor, Franken said some of the accusations were false, but acknowledged he couldn't continue. 
 
 "I know in my heart that nothing I have done as a senator — nothing — has brought dishonor on this institution," Franken declared. 
 
 The former comedian who made his name on "Saturday Night Live" is the latest to fall in the national wave of sexual harassment allegations, coming just two days after the resignation of Michigan Democratic Rep. John Conyers, who was the longest-serving member in the current House. 
 
 His rapid fall leaves Democratic Gov. Mark Dayton to appoint a replacement to serve until at least 2018, when the state would hold an election to finish Franken's six-year term ending in 2020. Dayton said Thursday he expected to do so "in the next couple of days." 
 
 Franken's exit gives Republicans an opening to expand their reach in an already favorable Senate map — two dozen Democratic senators are up for re-election next year, including 10 in states that President Donald Trump carried in 2016. That difficult math raises the question of whether Dayton will appoint a temporary placeholder or try to give a Democrat a running start for the 2018 campaign. 
 
 His lieutenant governor, Tina Smith, has been most frequently mentioned for the temporary appointment. Smith is a trusted former chief of staff for Dayton and also ran former Vice President Walter Mondale's brief Senate campaign in 2002 after Sen. Paul Wellstone died in a plane crash. But Smith previously announced she wouldn't run for governor next year, suggesting she would be no more than a caretaker. 
 
 Dayton could also look to a pair of fellow Democratic statewide elected officials: Attorney General Lori Swanson or State Auditor Rebecca Otto. Otto is running for governor and Swanson is thinking about it; either could conceivably turn into a Senate candidate. 
 
 While Dayton weighed his choice, eyes were already turning to the 2018 race, especially among Republicans. 
 
 Former Gov. Tim Pawlenty — the last Republican to win a statewide election in Minnesota, in 2006 — was seen as a potential heavyweight contender in the GOP. Another possibility, former Sen. Norm Coleman, quickly ruled himself out. 
 
 The timing of Franken's departure is uncertain. Fellow Minnesota Sen. Amy Klobuchar said Franken indicated he would officially step down sometime "by the end of the year." It's an abrupt end for someone who had been seen as coming into his own in Washington. 
 
 Franken, 66, had gained respect as a serious lawmaker in recent years and had even been mentioned in talk about the 2020 presidential race. But his downward spiral began Nov. 16, when Leeann Tweeden, now a Los Angeles radio anchor, accused him of forcibly kissing her during a 2006 USO tour in Afghanistan. 
 
 Other allegations followed, including a woman who says Franken put his hand on her buttocks as they posed for a photo at the Minnesota State Fair in 2010. Two women told the Huffington Post that Franken squeezed their buttocks at political events during his first campaign for the Senate in 2008. A fourth woman, an Army veteran, alleged Franken cupped her breast during a photo on a USO tour in 2003. 
 
 But Franken remained defiant in his parting speech, saying that "some of the allegations against me are simply not true" and disputing the details relayed by other accusers, as he had from the start. 
 
 And he pointedly noted that he was being forced out while Trump — who has been accused of worse offenses and bragged on a leaked "Access Hollywood" videotape of grabbing women by their genitalia — emerged unscathed. Trump and fellow Republicans have also endorsed Alabama GOP Senate candidate Roy Moore, who has been accused by multiple women of sexual misconduct with them when they were teens and he was a deputy district attorney in his 30s. 
 
 "I, of all people, am aware that there is some irony in the fact that I am leaving while a man who has bragged on tape about his history of sexual assault sits in the Oval Office and a man who has repeatedly preyed on young girls campaigns for the Senate with the full support of his party," Franken said. 
 
 ___ 
 
 Beaumont reported from Des Moines, Iowa. AP reporters Mary Clare Jalonick, Matthew Daly, Kevin Freking, Andrew Taylor and Alan Fram in Washington contributed.
– Democrats aren't in immediate danger of losing Al Franken's Senate seat. Minnesota's governor is a Democrat himself and will choose within the party for his temporary replacement. After that, however, all bets are off. And that's bad news for Democrats, explains the AP: Franken would not have faced re-election until 2020, and he likely would have kept his seat as the incumbent. Now, the seat will be up for grabs next year, and Republicans stand a decent chance of winning it. Franken's resignation may be a "total game-changer in terms of control of the Senate," Republican strategist Alex Conant tells NBC News. That's assuming, however, that Republicans field a strong candidate, and for now, eyes are turning to Tim Pawlenty, a former governor and 2012 presidential candidate. Pawlenty is in the private sector now as CEO of Financial Services Roundtable, and he has said he's done with politics. But that's not stopping supporters from raising his name. The situation is similar to the Republicans' plight in Alabama, where a once-safe seat is in play because of the Roy Moore scandal, notes Chris Cillizza at CNN. After assessing the whole Senate landscape, however, he concludes that it's still possible for Democrats to have control of the chamber by the time 2019 rolls around. As for Minnesota, the Minneapolis Star Tribune looks at the political calculus involved with the interim pick—caretaker or someone who would run again in 2018? For now, the best bet is that Gov. Mark Dayton will select Lt. Gov. Tina Smith, though the newspaper reports that Smith would likely not run in the 2018 election.