The significance of the problem became apparent in the same string of primaries that put Biden on the cusp of the nomination.

In Michigan — a state critical to Democrats’ efforts to reclaim their general election footing in the Rust Belt — just 2 of 5 Sanders backers said they would vote Democratic in November, regardless of who became the nominee, according to exit polls. Four in five said they'd be dissatisfied with Biden as the Democratic standard-bearer.

Though it's unclear how widespread or adamant the #NeverBiden contingent is — will they really stay home when the alternative is another four years of President Donald Trump? — the misgivings at least put the Biden campaign on notice that it has significant work to do to bring along Sanders' base.

There is certainly anecdotal evidence that for many progressives, Biden represents everything they dislike about mainstream Democratic politics. On “The Young Turks,” which draws millions of viewers, Krystal Ball, the former MSNBC host, said she couldn’t vote for Trump.

“But you can leave it blank,” she said, referring to the November ballot.

Ball said she is an "undecided voter" because "if they always can say, 'Look, you've got to vote for us no matter what, you've got no other choice,' then they're always going to treat us like this."

Paul Maslin, a top Democratic pollster who worked on the presidential campaigns of Jimmy Carter and Howard Dean, said that in their overtures to Sanders’ supporters, Democrats have “time, Trump and hopefully Bernie himself on our side.”

However, he said, “It’s a huge challenge.”

The November election is almost eight months away, and unlike in 2016, Sanders’ supporters don't have the hard feelings of superdelegates — the party bigwigs who clinched the nomination for Hillary Clinton — to overcome. This year, Sanders’ momentum was blunted not by those insiders but by black voters in the South and, following his victory in South Carolina, by the broader electorate.

Polls show Biden is also viewed more favorably now than Hillary Clinton was in 2016.

“At the end of the day, it’s Biden or Trump,” said Boyd Brown, a former South Carolina lawmaker and former Democratic National Committee member. “They’ll turn out.”

Still, some Sanders supporters see the consolidation of moderate presidential candidates and other elected officials around Biden as the establishment asserting its power over the grassroots. Trump has happily stoked the divide, declaring that the Democratic primary is “rigged” against Sanders, just as he did four years ago.

“The rationale for us is that our votes need to be earned and that we’ve been taken for granted, and the party never moves to us,” said Alyson Metzger, a 54-year-old writer and progressive activist in New York City who supports Sanders. “If they install Joe Biden, I will not vote for Biden. … This is not democratic what’s happening in the Democratic primary.”

For Metzger, not voting for Biden is a matter of conscience. For others, it is also strategy. On “Never Biden” Facebook pages and in Twitter threads, some activists argue that if Trump is reelected, Democrats will fare better in the next midterms and that the party will be more likely to nominate a progressive in 2024. If Biden is elected, they see eight years of centrist governance.

“I can’t vote for Joe Biden,” said Bryan Quinby, a left-wing podcaster in Ohio, saying that “it feels like the party doesn’t want us — the people who were pushing for Bernie Sanders and were enthusiastic about it.”

Noting that his vote was “never guaranteed for the Democrats,” he said that in November, “I think it just means I don’t vote for president.”

Of particular concern for Biden are young voters, including Latinos — the segments of the electorate that Sanders carried by a large margin. Evan Weber, national political director of the Sunrise Movement, a group of young climate change activists, said it is a “real possibility” that young liberals will stay home in November.

While stressing that he would vote for any Democratic nominee, Weber argued, “There’s lots of narratives about why Hillary Clinton lost the election, but one undeniable one is that she did not mobilize the young people who turned out for Obama in 2012 and especially 2008. We hear a lot from pundits about Joe Biden being Obama’s vice president and him being able to recreate the Obama coalition, but one of the core and critical components of the coalition was young people.”

Exit polls from this year’s primaries have found a stark age divide between Sanders and Biden supporters. In Michigan, almost two-thirds of voters under 45 backed Sanders, while Biden captured a third of them. It was even more lopsided among voters under 30: 4 in 5 voted for Sanders.

But Sanders has failed to turn young people out at rates he’d hoped, and among those over 45, Biden won two-thirds and Sanders 1 in 4.

Sanders, speaking at a news conference on Wednesday, reiterated that Trump “must be defeated” and that he “will do everything in my power to make that happen,” even as he vowed to press ahead in the nominating contest.

But while acknowledging Biden’s expanding lead in the primary, he warned, “You cannot simply be satisfied by winning the votes of people who are older.”

John Della Volpe, director of polling at the Harvard Kennedy School Institute of Politics, said Biden needs to make building a relationship with young voters “a central focus” of his campaign to beat Trump.

Recent Democratic nominees who have lost, such as Hillary Clinton and John Kerry, received about 55 percent of the youth vote, he said, whereas former President Barack Obama won 60 percent or more in his two races. “It’s not a coincidence that the youth vote crossed or met that 60 percent threshold in each of those campaigns that were successful," he said.

In the 2016 primary, Sanders dominated the youth vote against Clinton, winning more than 80 percent of voters under 30 in some states. When the general election came, some leading left-wing voices backed the Green Party’s Jill Stein.

This cycle, Sanders has said repeatedly on the campaign trail that he would support the nominee, regardless of who it is.

The consequence of even a portion of Sanders’ support evaporating instead of migrating to Biden could be significant in a close election against Trump. And the urgency for Biden to win over a constituency that Sanders has carried reliably has become plain in recent days.

Biden on Tuesday night thanked Sanders and his supporters directly “for their tireless passion,” saying, “We share a common goal. And together, we’ll defeat Donald Trump.”

The next day, his communications director, Kate Bedingfield, was on Fox News arguing that “there is a lot more that unites us than divides us."

"There is a whole lot about our message," she said, "that appeals to Sanders voters."