Hillary Clinton speaks in New York while her husband, former president Bill Clinton, applauds.

Nov. 9, 2016 Hillary Clinton speaks in New York while her husband, former president Bill Clinton, applauds. Melina Mara/The Washington Post

Democratic presidential nominee Hillary Clinton promised young voters Wednesday that she will champion issues they care about, especially the crushing cost of college, as she and former rival Sen. Bernie Sanders appealed to an age group that has never fully warmed to her.

“Is anybody here ready to transform America? You’ve come to the right place,” Sanders said at a joint rally with Clinton at the University of New Hampshire in Durham.

“Today I am asking all of you to think big, not small,” the senator from Vermont told a crowd that included many in the target audience of young people but also a large contingent of gray hair.

“We should and we will make public colleges tuition-free for families earning less than $125,000” annually, Clinton said.

Sanders praised Clinton for updating her platform to include tuition-free enrollment in public, in-state colleges for most families.

Audience members listen during the campaign stop by Hillary Clinton and Sen. Bernie Sanders at the University of New Hampshire. (Melina Mara/The Washington Post)

That pledge is a compromise with Sanders and his free-tuition proposal that was wildly popular with young voters; it was the capstone to a list of liberal positions Clinton outlined before a university audience.

“None of this will happen if you don’t turn out to vote. None of it will,” Clinton said.

Campaigning together in the state where Sanders defeated Clinton by 20 points in the February primary, Clinton and Sanders plugged what Sanders said would be a revolutionary way of paying for college.

“I want young people to leave school excited about the future,” Sanders said, “not being saddled with tens of thousands of dollars in student debt.”

Clinton announced the shift in her platform in July, ahead of the Democratic National Convention. During her primary fight, Clinton had proposed that community colleges be tuition-free for all working families.

The original cost of Clinton’s college plan was $350 billion over 10 years. A Clinton aide said the expansion will raise the cost by more than $100 billion. Any such plan would require congressional approval.

“The truth is, it is an expensive proposal,” Sanders acknowledged. “But I will tell you what is even more expensive, and that is doing nothing.”

Hillary Clinton's campaign said that retired Republican senator John Warner of Virginia will endorse Clinton, marking the first time he has endorsed a Democratic presidential candidate and adding to the list of high-powered GOP figures stepping away from Donald Trump. (Video: WUSA9 / Photo: Kate Patterson/For The Washington Post)

Clinton and Sanders spoke in front of huge new signs promising “debt-free college,” the most visible element of Clinton’s effort to attract younger voters. They led a wonky discussion that included preselected questions from a few members of the audience.

Republican nominee Donald J. Trump’s name did not come up much, and Clinton made only passing reference to the widely watched debate between the two presidential nominees on Monday.

Instead, the discussion focused firmly on economic, social and environmental issues of prime interest to younger voters. Clinton spoke of her own youthful experiences borrowing and paying off law school loans and afterward being able to take a public-interest job that paid little.

Sanders nodded and smiled as she spoke, and she smiled as he riffed on favorite themes from his primary, including the role of special interests in politics.

“She’s really underperforming with millennials,” said Ben Tulchin, who was a pollster for Sanders during the primary. “It’s obviously helpful for him to vouch for her, and say her economic message aligns with his.

“Bernie was a rock star with them, and I don’t think she’ll be able to replicate his success,” Tulchin said. “But she can keep making progress.”

After rolling out the joint college-affordability plan in July, Clinton didn’t keep it in the spotlight like she should have, Tulchin said.

“She’s really got to lean into it,” he said. “It sounds like the campaign gets it now, but they get it because it’s a crisis.”

Sanders’s victory in the symbolically important first primary of the presidential season here sealed his importance as a liberal alternative to Clinton and demonstrated his appeal to young voters. Voters under 35 were a critical trouble spot for Clinton then, and they remain so now — despite the efforts of Sanders, liberal heroine Sen. Elizabeth Warren (D-Mass.) and first lady Michelle Obama to win them over.

Obama spoke Wednesday in Pennsylvania; Warren campaigned on Clinton’s behalf on the same New Hampshire campus last week.

The RealClearPolitics average of recent state polls shows Clinton with a comfortable five-point advantage over Trump in New Hampshire, but it also shows the strength of third-party candidates in a state that rewards iconoclasts. Clinton would be the choice of 42.7 percent of voters in polls taken this month, Trump 37.3 percent and Libertarian Gary Johnson 13 percent. Green Party candidate Jill Stein earns 2.7 percent of support.

A Quinnipiac University poll this month shows Clinton with about 55 percent of support among voters 35 and younger and leading Trump by about 20 points in a head-to-head contest, but her support among this group plummets to 31 percent when third-party candidates are included.

This is only the second time that Sanders and Clinton have held a joint event since the primary season ended. The first was his endorsement of her in Portsmouth, N.H.

“I’m sure there will be more of these,” said Jeff Weaver, who was Sanders’s campaign manager.

Weaver said the aim isn’t as much for him to travel to states he won as to motivate constituencies that responded to his message.

“There are a number of states he can help with,” Weaver said. “But more than the states, it’s the constituencies he can help with.”

During an appearance Sunday on CBS’s “Face the Nation,” Sanders said he wasn’t sure why Clinton was not doing better with the younger demographic.

“But I think the antidote is that she has got to make it clear to not only the millennials but every American the difference that she has, not just on personality issues, which is what the media focuses on, but the real issues impacting the middle class and working families of this country,” Sanders told host John Dickerson.

John Wagner in Washington contributed to this report.