The campaign also pointed to the geographic sweep of Mr. Biden’s victories on Tuesday, particularly in Michigan: He won every county there, including those home to the University of Michigan and Michigan State University, which Mr. Sanders won by double-digit margins in 2016, though Mr. Biden’s margins there were much smaller than in other parts of the state.

From the start, Mr. Biden’s argument for his candidacy has centered explicitly on a return to “normalcy” after four years of life under Mr. Trump. And that is not what many young voters are looking for.

“The Republican Party has moved farther to the right than Democrats have moved to the left, which leaves this huge vacuum, I feel, for younger people who actually want to be a part of a truly progressive movement,” said Hajar Khalid, 20, a student at Columbia University.

Ms. Khalid said she supported Medicare for all and would feel somewhat better about voting for Mr. Biden, should he win the nomination, if he had a more comprehensive health care plan or if he chose a truly progressive running mate.

“I want to say that despite all of this, I will still vote for the Democratic Party in the general no matter who it is,” she said. “But it’s really hard to say that when the other candidate is Joe Biden, who doesn’t have any real attractive attributes and whose main message seems to be: ‘I won’t change anything to help progressive causes, really, but at least I’m not Trump.’”

Beyond the disconnect between young progressives’ ideals and Mr. Biden’s more moderate proposals, there is also a deeper problem: While Mr. Sanders has actively engaged with young people, Mr. Biden has never given much indication that he is seeking their votes at all. At times, he has appeared openly disdainful or condescending toward them.

Groups that work with young voters said winning their votes would require speaking more to the policies they want to see enacted, but also considering them in event planning, advertising and communications. And it could mean enlisting surrogates who have more credibility among young voters than Mr. Biden does.