(CNN) Walk through campus at East Carolina University in Greenville, North Carolina, and ask students whom they plan to vote for, and Hillary Clinton's struggle to win millennial voters is immediately clear.

Take three students who were standing together outside a student center. None was a Republican. None likes Donald Trump. But only one said he would vote for Clinton.

Courting young voters is a central part of the Clinton campaign's grassroots efforts in the final weeks before Election Day, and for good reason.

Young voters fueled President Barack Obama's victories. He got 60% of 18-29-year-olds in 2012, a demographic that was nearly 20% of the vote. This week Clinton predicted it could grow to 25% this year.

To lure them to her campaign, team Clinton has ramped up efforts to engage millennials -- especially Bernie Sanders supporters -- by bringing on several of the Vermont senator's former campaign aides to mobilize voters under age 35.

The campaign has also hired youth directors in swing states across the country to lead the charge registering and courting voters on more than 280 college campuses.

Lillie Catlin, Clinton campaign's North Carolina Youth Director, told us that -- not surprisingly -- a big part of their efforts rely on social media: Facebook, Instagram, Twitter, Snapchat -- any and all tools millennials use to communicate with one another, the campaign uses too.

"We have a really exciting digital program and a lot of it's being run by our campus organizers themselves," Catlin said.

"We can't, you know, knock on every dorm and students move every year and so it's, you know having those kinds of conversations but through our text lists and on Twitter," she added.

Still, polls show Clinton is under performing with millennials, so the campaign is deploying high-profile surrogates. Sanders is lobbying his army of young supporters, campaigning along side Clinton this week. Michelle Obama too is targeting millennials, and her husband is planning to help Clinton by going hard after this critical part of his winning coalition.

Chelsea Clinton also is hopscotching to college towns. We caught up with her during a visit to North Carolina this week and asked why her mother is having such a challenge exciting millennials. She conceded, "we need to be doing a better job of collectively making the case at what's at stake in this election," noting that only about 55% of 18-24-year-olds were even registered to vote at the beginning of 2016.

Chelsea Clinton headlined a forum for students at East Carolina University, talking mostly about the college affordability plan her mother worked on with Sanders after he dropped out of the primary race. During Q-and-A with students, Erick Jenkins told her that he is a Sanders delegate who is now supporting Secretary Clinton, largely because of her college plan.

But another Sanders delegate, Zackary Pate, asked what "moral ground" Hillary Clinton has to stand on to run as the Democratic nominee.

Chelsea Clinton responded by politely thanking him for being a delegate, and urging him to listen to Sanders, who is now making the case for Hillary Clinton.

During another back and forth, Pate, who really came to protest, walked out as Chelsea Clinton was speaking, carrying a sign for Green Party candidate Jill Stein.

Afterward Pate told us he believes the Democratic primary was rigged for Clinton and that he's going to write in Stein's name on the ballot -- not because he thinks she can win -- but as a way to empower a third party.

"This is to make a statement that says that every political party and every voice in this country should be heard, and have fair access to people, to media, to debates, to ballot access, to everything that matters and makes us a democratic nation," he said.

Losing millennials to third-party candidates is a dire concern for Clinton. A Quinnipiac University national poll this month found that 44% of voters 18-34 said they would vote for Stein or Libertarian Gary Johnson, while only 31% said they would back Clinton.

Stumping for Clinton this week, Michelle Obama argued a vote for a third party is really a vote for Donald Trump.

"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," said the first lady.

At the end of the day, Clinton campaign aides say they think the best people to make this case to millennials are their peers.

Anniken Williams, the Vice President of North Carolina College Democrats who sat on stage with Chelsea Clinton this week, admitted afterward that her experience getting friends on the Clinton train has been "pretty difficult."

She says a lot of people she knows have told her they will vote for a third-party candidate.

"To me when you vote for a third party -- I've studied this, I'm a political science major -- when you vote third party you're throwing your vote away," Williams said.

She told us that she has successfully changed a few minds making that case around campus.

Back at Clinton campaign headquarters in New York, they collect all kinds of data from their vast grassroots efforts to try to figure out if their efforts to lure millennials is working.

But there is an unlikely barometer beyond the fancy metrics that top advisers say is most telling: tracking online sales of Hillary Clinton campaign merchandise. The cooler she is to young people, the more they tend to buy her campaign hats and t-shirts.