[Blitzer:] While Republican frontrunner Donald Trump faces a revolt within the party establishment, Democratic frontrunner Hillary Clinton faces a problem of her own making. The inquiry into her e- mails as secretary of state. Bernie Sanders is trying to gain ground ahead of Sunday's CNN Democratic presidential debate. Our senior Washington correspondent, Jeff Zeleny is covering the Democratic campaign for us. What's the latest out there, Jeff?
[Jeff Zeleny, Cnn Senior Washington Correspondent:] Wolf, I can tell you Bernie Sanders is traveling to a collection of states that are trying to make up for some lost ground earlier in the week. He held a rally a short time ago here in Lincoln, Nebraska. That's a place where Democratic presidential candidates seldom visit. He did not stop drawing contrasts and distinctions with Hillary Clinton. Did not make up any comments about her e-mail, but he's all after one thing, Wolf. The state's 25 delegates on Saturday.
[Zeleny:] Bernie Sanders had a message today for Democrats. The primary isn't over yet.
[Sen. Bernie Sanders , Presidential Candidate:] What a fantastic turnout this afternoon. Thank you so much.
[Zeleny:] Yet the campaign trail to himself from Michigan to Nebraska to Kansas, hunting for delegates in his uphill fight with Hillary Clinton. After basking in the glow of Elton John and Katy Perry Wednesday night at one of the Clinton campaign's biggest fundraisers.
[Katy Perry, Musician:] This is it.
[Zeleny:] She spent the day out of public view as questions about her e-mail returned. A former aide was granted immunity and will talk to the FBI about the private server she used as secretary of state. It became instant fodder today for Republicans.
[Donald Trump , Presidential Candidate:] Assuming she's not arrested for the e-mail situation, which is so terrible.
[Zeleny:] Again and again, Clinton has defended how she handled classified information.
[Hillary Clinton , Presidential Candidate:] It is just not something that, you know, is going t have any lasting effect and I'm not at all worried about it.
[Zeleny:] But the Justice Department inquiry still hangs over her campaign. Sanders once again steered clear but he wasted no time going after other parts of her record, starting with the Keystone Pipeline, which would run straight through Nebraska.
[Sanders:] There is a candidate out there running for the Democratic nomination who was a little bit wobbly about the Keystone Pipeline.
[Zeleny:] And on trade he called her positions a disaster for Michigan.
[Sanders:] She was very, very wrong and millions of families around this country have been suffering as a result of those disastrous trade agreements.
[Zeleny:] He told CNN he has no time for Democrats who say he should tone down his rhetoric against Clinton.
[Sanders:] In many ways Democrats can say what they want. We're in this race to win it. I don't run negative campaign ads but I do think it is appropriate that in a campaign you distinguish your differences with your opponents.
[Zeleny:] He's fighting to win the Nebraska and Kansas caucuses on Saturday and Maine on Sunday.
[Sanders:] On the other hand, maybe Nebraska is not quite so conservative.
[Zeleny:] Still it's an uphill climb for Sanders. In pledged delegates he has 405 to Clinton's 606. But when you factor in super delegates, Clinton's support soars. 1,074 to 426. And Wolf, when we talked to Senator Sanders this afternoon, he said he has no intention of either lightening up and certainly not leaving this race. The Sanders campaign believes by the end of this week, and they will have three more states in their win column. The Clinton campaign does not disagree. She believes she will only win the Louisiana primary on Saturday. Wolf, of course this is a race about delegates and these states are all small numbers of delegates but we are entering a very busy stretch here. That key debate on Sunday in Flint, Michigan, followed by another debate next Wednesday, this could be one of the most important stretches of this Democratic campaign Wolf.
[Blitzer:] You're absolutely right. Jeff Zeleny, reporting for us, thank you. Coming up, 2012 Republican presidential nominee Mitt Romney slamming 2016 presidential frontrunner Donald Trump as a phony and a fraud. Does Romney have a secret plan to deprive Trump of the nomination? And breaking news. A day after the United Nations imposes tough new sanctions, North Korea fires off a salvo of missiles and Kim Jong-Un orders nuclear warheads be ready for use.