Speaking to a gathering of campaign supporters in February, Clinton said some of Sanders' followers "are new to politics completely. They're children of the Great Recession. And they are living in their parents' basement."
She continued: "If you're feeling like you're consigned to, you know, being a barista, or you know, some other job that doesn't pay a lot, and doesn't have some other ladder of opportunity attached to it, then the idea that maybe, just maybe, you could be part of a political revolution is pretty appealing. I think we all should be really understanding of that."
The Democratic nominee -- who at one point said she didn't want to "overpromise" and "be clear about the progress we can make" -- questioned if Sanders' supporters were adequately informed.
"There's just a deep desire to believe that we can have free college, free health care, that what we've done hasn't gone far enough, and that we just need to, you know, go as far as, you know, 'Scandinavia,' whatever that means," she says in the audio. "And half the people don't know what that means, but it's something that they deeply feel."
She also said: "I don't think you tell idealistic people, particularly young people, that they've bought into a false promise. You try to do the best you can to say, 'Hey that's his view. That's what he's offering you. But here's another way, where actually we can achieve a lot of what we have said starting Day One, and make a real difference in people's lives.'"
In a statement, the Clinton campaign did not dispute the audio's authenticity.
"As Hillary Clinton said in those remarks, she wants young people to be idealistic and set big goals. She is fighting for exactly what the millennial generation cares most about -- a fairer more equal, just world," said Glen Caplin, a campaign spokesman. He added: "She's inspired by the optimism and the drive of this generation and Sanders supporters across the country -- and they've helped her craft and promote the most progressive platform in Democratic party history."
A message left with Sanders' campaign manager, Jeff Weaver, was not immediately returned. But Mike Casca, a Sanders spokesman, tweeted
that Clinton's comments were sympathetic and understanding of the supporters' frustrations.
Clinton is not polling as well with millennial voters as President Barack Obama did in 2008 and 2012, with many backing Libertarian Gary Johnson and Green Party candidate Jill Stein.
But following the Democratic convention, the Clinton campaign has enlisted surrogates such as Sanders, Massachusetts Sen. Elizabeth Warren, first lady Michelle Obama and Chelsea Clinton to stump for Clinton and convince young people to back her.
Clinton also has embraced some of the policy positions that made Sanders such a hit with millenials, such as a $15 federal minimum wage and greater taxpayer funding for health care.
Trump on Saturday night leaned into the issue as he made his latest play to earn the votes of Sanders' supporters.
Speaking at a rally in Manheim, Pennsylvania, Trump accused Clinton of "demeaning and mocking Bernie Sanders and all of his supporters" and argued that Clinton was calling Sanders supporters "ignorant."
Trump argued that Clinton delivered her comments in a "really sarcastic tone because she's a sarcastic woman."
"To sum up, Hillary Clinton thinks Bernie Sanders supporters are hopeless and ignorant basement-dwellers," he said.
Trump said he believed many of Sanders' supporters would cross over and vote for him, rather than Clinton, particularly because of their shared distrust on free trade.
Echoes of past Clinton criticisms
Clinton has previously suggested that Sanders was promising his supporters too much.
In January, she told voters in Iowa that she agrees with "very aspirational" candidates but is "also interested in getting things done."
"Now, I know some of the folks running are very aspirational, and I agree with that. We should all be aiming high. But I'm also interested in getting things done," she said in January. "We can't wait to get a lot of new jobs. We can't wait to get incomes rising. We can't wait to make sure that we are moving forward together. And I also know something about how to make sure that the financial markets, Wall Street, doesn't wreck our economy."
The next month, Clinton told New Hampshire's WKXL that "overpromising" from candidates is one of the main reasons many people are disappointed in politics.
"It worries me because I think overpromising is one of the roots of a lot of disappointment and the real dismissal of politics, on all sides of the political spectrum. And what we need now is to underpromise and overdeliver," she said.
"What I'm trying to make the case for -- which is not as attractive -- it's not the romantic, dramatic appeal that others can make," Clinton added. "But I think it is what the country will eventually respond to, because if you say, as I do, I'm going to fight to make real change, to build on progress, to go further, but I will not promise what I cannot, and know I cannot, deliver."