Washington ( CNN ) -- 2010 has opened on an unsettling note for Democrats in the U.S. Senate . Connecticut 's Chris Dodd and North Dakota 's Byron Dorgan -- two longtime power players in the chamber -- have announced their intention to step down at the end of the year . If history is any guide , the party now faces an uphill struggle to maintain its 60-seat supermajority . Failure to do so could have serious ramifications for President Obama as he tries to look past the health care debate and tackle global warming , ballooning budget deficits and a range of other politically contentious issues . The fight over health care reform has clearly demonstrated that 60 votes is now the minimum threshold for passing major legislation through the Senate . Anything less leaves the majority party at the mercy of a minority increasingly willing to employ the filibuster to grind the legislative gears of the Senate to a halt . Democrats currently have exactly 60 members in their caucus ; Republicans have 40 . The GOP knows that presidents typically lose House and Senate seats during midterm elections , and is banking on that fact to block much of Obama 's agenda . `` Midterm elections are usually low-turnout affairs , '' noted CNN polling director Keating Holland . `` Conservative Republicans , however , have been energized by their strong opposition to Obama 's agenda . They 're likely to vote this November . Many liberals , on the other hand , do n't feel Obama has pushed strong enough on their issues . It 's unclear at the moment if they 'll be as likely to show at the polls . '' This year , Democrats must defend 19 seats , including an opening that occurred when veteran Massachusetts Sen. Ted Kennedy died . A special election for Kennedy 's seat -- widely expected to be won by the Democrats -- will take place January 19 . Republicans are defending 18 seats . The GOP , according to many analysts , is now placing a high priority on picking up Democratic seats in Colorado , Delaware , Illinois , Nevada , North Dakota and Pennsylvania . The Illinois and Delaware seats were once held by Obama and Vice President Joe Biden , respectively . Obama 's replacement , Sen. Roland Burris , was immediately tainted after being tapped to fill Obama 's vacancy by Rod Blagojevich , the state 's scandal-plagued former governor . Burris is not seeking election in his own right . Biden 's replacement , Sen. Ted Kaufman , is likely to step aside in favor of the vice president 's son , Delaware Attorney General Beau Biden , analysts believe . Delaware 's longtime GOP congressman , Mike Castle , is being touted as the likely Republican nominee . Senate Majority Leader Harry Reid , D-Nevada , is struggling to manage Obama 's more progressive agenda while also seeking a fifth term from his traditionally conservative state . Republicans , however , are having a tough time unifying behind an opponent . At the same time , five-term Pennsylvania Sen. Arlen Specter remains a high-profile target for conservatives enraged by his decision to switch from the Republican Party to the Democratic Party last year . Before he can get to the general election , Specter will face a tough primary challenge from Rep. Joe Sestak , a former admiral . Also in the GOP 's crosshairs is two-term Arkansas Democrat Blanche Lincoln , who recently cast a tough political vote in favor of Reid 's health care bill . Dodd 's decision to retire , however , is widely believed to be beneficial to Democratic chances in largely liberal Connecticut . The state 's popular Democratic attorney general , Richard Blumenthal , declared his candidacy a few hours after Dodd announced he was stepping down . Democratic leaders , for their part , are placing a high priority on picking up GOP seats in Kentucky , Missouri , New Hampshire , and Ohio . Republicans have been stung by incumbents ' decisions to retire in all four states . Louisiana , a more conservative state , has attracted the interest of Democratic strategists . First-term GOP Sen. David Vitter has been tarred by his 2007 admission that he had a relationship with a prostitute . This year 's race in Florida has also become an increasingly high-profile contest , largely due to Republican infighting . GOP Sen. George LeMieux , who was appointed to the Senate after former Sen. Mel Martinez unexpectedly stepped down last year , is not seeking a full term . The state 's GOP governor , Charlie Crist , was widely expected to take the seat , but is now facing an unexpectedly strong conservative primary challenge from former Florida House Speaker Marco Rubio . Republicans are still , however , favored to hold the seat . CNN 's Mark Preston and Paul Steinhauser contributed to this report .
Democrats need 60-seat supermajority in Senate to get major bills passed . Sens . Byron Dorgan and Chris Dodd announced This year they wo n't seek re-election . Senate targeting vulnerable seats , including those vacated by Obama and Biden . Flipping could go both ways , with Democrats picking up some GOP seats .