Durham, New Hampshire(CNN) Outside of a Hillary Clinton rally at the University of New Hampshire Wednesday afternoon, a long line of people -- many of them backpack-wearing students -- wrapped around the block, and a man held a homemade sign that read: "If there were ever a time for a third party vote, the time is now."

It could only mean one thing: Bernie Sanders was in town.

In the final homestretch of the 2016 presidential campaign, Clinton is making an aggressive push to win over millennial and liberal voters who shunned her during the primaries and aligned themselves, instead, with the famously raspy, populist senator from Vermont.

Taking the stage together with Clinton in a state where he beat her by more than 20 points, Sanders urged the crowd to get behind his one-time rival: "It is imperative that we elect Hillary Clinton as our next president."

But even with the popular senator hitting the campaign trail for her, Clinton has her work cut out.

Inside a packed gymnasium here, attendees under the age of 30 -- a demographic group that Clinton is struggling with -- expressed lingering regret about Sanders' failed candidacy, and more importantly, nagging reservations about the Democratic nominee.

Of the first seven people CNN spoke with, six supported or leaned towards Sanders during the primaries, while one voted for Ohio Gov. John Kasich.

Christian Flores, a 16-year-old junior at Exeter Academy, is too young to vote, but came to Wednesday's rally with the hopes of getting a photo with Sanders. Flores is a member of his school's Democratic club, and credited Sanders for his new interest in politics.

"When Bernie started saying things that really appealed to my generation, that's when I really started to get involved," said Flores, the son of immigrants from Ecuador. "Secretary Clinton, after the fairly contested primary, comes out and magically adopts all of (Sanders') policies -- I think there's a little bit of sense of doubt within my generation."

If Flores were old enough, he said he would likely vote for Clinton or Green Party candidate Jill Stein in November.

Emily Bailer, an 18-year-old student at St. Anslem College in Manchester, was drawn to Sanders during the primaries but is now volunteering with the Clinton campaign. She said her initial doubts about Clinton had mostly do do with her emails scandal.

"I had some issues trusting her," Bailer said.

But Bailer added that just like herself, many of the ex-Sanders supporters she knows appear to be coming around to Clinton.

"I don't know, the people I've talked to -- they're starting to change their minds and are starting to support her because they know the alternative," she said.

That alternative is Republican presidential nominee Donald Trump -- perhaps Clinton's most powerful tool for bringing in new supporters into the fold.

One man in the crowd -- 26-year-old Brian Zukas -- supported Kasich in the primaries. A graduate student at the University of New Hampshire and a registered Republican, Zukas said he will vote for Clinton in the fall because she is the only acceptable non-Trump option.

"At this point, I can't not vote," he said.

A major challenge for Clinton as she looks to win over Sanders supporters is to stop them from defecting to other third-party candidates.

Outside of the Clinton-Sanders joint rally in Durham on Wednesday was a small group of Stein supporters. One held up a sign opposing Citizens United, the Trans-Pacific Partnership trade deal and fracking — issues that were central to Sanders' insurgent campaign.

Younger voters overwhelmingly backed President Barack Obama in 2012, handing him a 29-point lead over Republican nominee Mitt Romney, according to exit polls. But the enthusiasm that young voters, including many first-time voters, showered on Obama, has not transferred over to Clinton.

Drawn to Sanders' positions of college affordability, universal health care and climate change, millions of millennial voters supported the senator during the Democratic primary. The group has been slow to gravitate towards Clinton, even though Sanders officially endorsed her at a New Hampshire event in July.

Clinton spokeswoman Jennifer Palmieri told reporters Wednesday afternoon that Sanders has "already been effective in making that argument that if you want to defeat Donald Trump you need to vote for Hillary Clinton."

According to Clinton and Sanders aides, the Vermont senator will be an active surrogate in the final weeks of the 2016 election, and more campaign events are being scheduled. One Sanders aide said the senator will likely visit Iowa, Michigan, Ohio and Pennsylvania.

On Wednesday, Clinton called Sanders a friend and heralded him as "one of the most passionate champions for equality and justice that I have ever seen."

"Bernie's campaign energized so many young people. And there is no group of Americans who have more at stake than young Americans," Clinton said.

Without dwelling on their at-times contentious primary, Clinton said she was "proud" of her race against Sanders and even prouder of the fact that they "began to work together how we can take the issues we agreed on."

Neviah Eaton, a 24-year-old mother of a 2-year-old, voted for Sanders during the primary, and while she is still a Sanders fan, will vote for Clinton in November.

"A lot of the younger voters were really set on Bernie. We've got the 'Bernie or bust' -- I don't really agree with that," Eaton said. "Because the more people we have voting for Hillary, the less likely that Trump because president."