"Some are new to politics completely. They're children of the Great Recession. And they are living in their parents' basement," Clinton said at the home of former Ambassador Beatrice Welters. "They feel they got their education and the jobs that are available to them are not at all what they envisioned for themselves. And they don't see much of a future."
"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, and half the people don't know what that means, but it's something that they deeply feel," Clinton told the donors.
Some Sanders supporters have taken to Twitter voicing their disagreement with the message, but Sanders' deputy communications director, Mike Casca, and the Clinton campaign have pointed out how the tone of Clinton's comments is not in any way mocking Sanders supporters, as some have reported.
Clinton "wants young people to be idealistic and set big goals," her campaign spokesman Glen Caplin said. "She is fighting for exactly what the millennial generation cares most about — a fairer, more equal, just world."
But Trump heard something different in Clinton's words, describing her in one tweet as being "nasty to Sanders supporters behind closed doors."
Trump's campaign has linked Clinton's words at the February fundraiser to her statement at another fundraiser last month, when she referred to half of Trump's supporters as a "basket of deplorables."
His campaign manager, Kellyanne Conway, asked on Twitter, "Who else does @HillaryClinton ridicule behind closed doors for $$$?"
Sanders will be a guest on ABC's "This Week" on Sunday.
ABC News' MaryAlice Parks and Liz Kreutz contributed to this report.