Transcript for Bernie Sanders, Hillary Clinton Team Up in NH
We need not remind you Bernie sanders spent the presidential primaries building a revolution, battling Hillary Clinton, suggesting she was unqualified, bad for trade, and in the pocket of Wall Street. So why is he campaigning so passionately with her now? Here's ABC's David Wright. Reporter: Today in new Hampshire, a special guest out on the campaign trail. Senator Bernie sanders! Reporter: Senator Bernie sanders, who put up such a fierce fight for the democratic nomination. Today hit the stump alongside his old rival. It is imperative that we elect Hillary Clinton as our next president. Isn't this one of the strangest elections you've ever seen? Reporter: After that long and bitter primary, they may seem 94 like frenemies than friends. But the truth is especially in states like New Hampshire, Clinton needs sanders voters. Clinton has yet to convince many die-hard Bernie sanders fans, fully half of them don't think she's honest. If I think that Bernie has a chance with my vote of becoming our next president, then he will get my vote. Otherwise my vote will go to Dr. Jill stein. Reporter: The republicans have their own set of problems, not least the effort to explain trump's performance at Monday's debate. I think my strongest asset, maybe my far, is my temperament. Reporter: One high-level trump adviser pushed back on the notion the candidate didn't prepare adequately, noting his failures were more a lack of execution than preparation. The conaccepts consensus seems to be growing inside the trump campaign it was not his best night. The democrats aren't counting their chickens just yet. One of their concerns, supporters of the Vermont senator who gave Clinton such a run for her money. It's clear the Bernie sanders candidacy created a permanent problem for Hillary Clinton. She is not winning millennial voters in anything like the same Numbers that Barack Obama did. Bernie sanders is now out on the trail. If he can convince those young people who went for him to go for her, that will make an enormous difference in November. Happy to be spending a few more nights at home? Reporter: Today we caught up with sanders in New Hampshire as he tries to convince his voters to come over to Clinton. Senator, we've met any number of people for whom even at this stage it's Bernie or bust. What's your message? My message is that people have got to get beyond personality. And they've got to take a hard look at the issues that impact the lives of middle-class and working-class people. Hillary Clinton's positions are far, far, far superior to Donald Trump. Do you see this choice in November as a choice of the lesser of two evils? No, I really don't. Secretary Clinton and I disagree on a lot of issues. That was what we campaigned on for a year. So I'm not going to deny that for a second. But I think if you look at her proposals, issue by issue, a number of them are very Progressive. But you talked about this as a political revolution. And you brought a whole lot of people into the process who otherwise wouldn't have cared. Now many of them are saying, you know what I'm going to sit this one out, or I'm going for a third-party candidate who's more aligned with my values. How can you blame them? I'm not here to blame anybody. Everybody has the right to vote for whomever they want. All I can say is, Donald Trump must not be elected president and I intend to do everything I personally can to make sure that does not happen, and that means I'm going to campaign very hard for Clinton, not only here in New Hampshire but all across this country. Reporter: It's a problem Clinton faces in several of the key battlegrounds. Not just New Hampshire. The polls in those states showing her slightly ahead. But too close for comfort. Millennials overwhelmingly supported sanders in the primaries. Partly because of his Progressive message. The nation based on social justice, economic justice, racial justice -- Reporter: Partly because of his anti-establishment views. It is time for a political revolution. Reporter: Millennials are a huge group, if you can get them out to the polls. Roughly 30% of the electorate. In 2012, under-25s accounted for a slightly bigger share than Latinos. But now that their favorite candidate is out, it's not clear Clinton will be an acceptable second choice. If it came down to it tomorrow -- I don't even know who I would vote for. I will probably end up voting for Clinton. I mean -- The lesser of two evils? Exactly, yeah. Are you going to vote? Yeah. Bernie sanders is urging his people to vote for Hillary. Does that carry any weight with you? No, not really. It's very hard to transfer votes. Endorsements don't really work very much these days. But Bernie sanders does have a very loyal cadre of young supporters. And perhaps they will listen to him, who knows. Reporter: It's not so much these young voters are struggling to choose between Clinton and trump. For many the dilemma is more like Clinton or none of the above. Maybe a third-party candidate. Or maybe just sit this one out. That's why Michelle Obama was out today wooing young voters in Pennsylvania. And if you vote for someone other than Hillary, or if you don't vote at all, then you are helping to elect Hillary's opponent. Reporter: Here in new Hampshire, the scenario that keeps democrats like state party chair Raymond Buckley up at night is what happened to Al gore in 2000. Is that your nightmare scenario this year? The nightmare of 2000 haunts every one of us every day. For just a few thousand votes, Al gore would have been elected president. You believe if he had spent more time in New Hampshire, he wouldn't have had to worry about Florida? Absolutely. Reporter: The theory goes that a few thousand votes for Ralph Nader cost gore new Hampshire's four electoral votes. If he'd won those four votes, the Florida recount might never have been an issue. The question now, if those sanders voters sit this one out, could it happen again to Hillary Clinton? If she were to lose, does any part of you worry you would go down at the Ralph Nader of 2016? No, I don't think so. The logical corollary is we should not have competitive elections, that we should point somebody to run, and anybody runs against them would be responsible. I think more people think my candidacy made her a stronger candidate against trump. Reporter: Even so, Bernie sanders plans to be out on the trail in the battleground states. No longer his own man. Now he's with her. I'm David Wright for "Nightline" in Durham, New Hampshire. Our thanks to David. With 40 days to the election, stay on top of all the political news. Download the ABC news app and get live streaming of breaking news from the campaign trail, magic button. Next, a hopeless romantic at Yankee stadium dropped the ball
This transcript has been automatically generated and may not be 100% accurate.