Is this a case of the usual momentary sour grapes, or something more significant? In the wake of Joe Biden’s sudden — and largely fortunate — rise to dominance in the Democratic presidential primary, Politico reports that a new reaction threatens his ability to unite the party behind his candidacy. Meet the #NeverBidens, the #WriteInBernie bros, and the perhaps-coming #DemExit2020:

There’s no question it’s been a banner two weeks for Biden. But lurking in the background of his sudden ascension to all-but-presumptive nominee is evidence that at least some Bernie Sanders supporters might not migrate to him in November, weakening him in the general election.

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.

In every primary cycle, there is a period of time that can be fairly described as A Great Wailing and Gnashing of Teeth. People invest themselves heavily in primary fights, to the point where their side becomes all sweetness and light and their opponents become a cross between Darth Vader and the Three Stooges. In the immediate aftermath of that collapse, people react irrationally and declare themselves unalterably opposed to the primary winner no matter what happens, even if that means voting against their ideological interests.

Usually that dissipates by the time the conventions roll around, but not always. We still have a #NeverTrump movement, for instance, although its significance is small enough that it couldn’t even generate a primary challenge in this cycle. That split didn’t keep Trump from winning it all in 2016, although it might have against a stronger opponent than Hillary Clinton.

In this case, the split may well be more significant. Bernie Sanders’ voters are less likely to identify as Democrats (or at least as strongly) as supporters of Biden’s other primary voters, and that means they’re more likely to sit out the election. They are also more ideological than partisan as a result, which means that they aren’t as likely to take half a loaf over none at all. Some will, but not the true believers, and they make up a solid core of voters that Biden will need in November. Unfortunately, their ideological needs would put them at direct conflict with the centrism that Biden needs to embody in order to win a national election against Trump, which puts Biden between a rock and a hard place.

That tapdance would tax the capabilities even of the most skilled politician. As I write in my column for The Week, however, Biden’s not much of a dancer, and is more lucky than good:

The quality of Biden’s sudden good fortune raises another question about his viability, too. Until Sanders started offering arguments about the good side of Fidel Castro and communist China, Biden had trailed in a number of Super Tuesday states. After winning South Carolina and having the remaining center-lane candidates drop out, Biden began winning in places he’d barely visited — Minnesota, Massachusetts, Maine, and arguably Texas — where Sanders had been presumed to be ahead. Polls suggested that Sanders would score a huge delegate advantage over Biden in progressive California, but Biden finished within seven points of Sanders and with just 50 fewer delegates.

The impression this leaves is of a candidate who is winning by default, not by skill or argument. Exit polls favoring electability over policy amplify that impression. Sanders must believe that to be true, too, which is why he’s defying the delegate math for a few more days to take a direct run at Biden on stage. All Biden has to do at Sunday’s debate is survive it without doing too much damage to his own case. Sanders can only win now if Biden stumbles so badly as to make himself the more dangerous choice in a general election, and it will take a historic stumble to make the Fidel apologist the safe choice.

However, Biden’s performance thus far should raise serious concerns about his ability to compete against President Trump. It’s true that Trump largely won four years ago because of the incompetence of Hillary Clinton’s campaign and the disgust she inspired among voters, as Chait notes. This time around, though, Trump’s record primary turnout while running largely unopposed hints at the kind of voter-turnout organization that he largely eschewed in 2016. If Sunday’s debate is Bernie’s last stand, it may also be the last chance Biden has to prove he can actually compete on his own terms in a presidential contest. A default candidacy may not suffice in the general election.

Even if Biden fumbles his way through Sunday’s debate, his nomination is all but assured. His ability to hold the party together is still a big question, and it won’t take a major #DemExit2020 to send Biden permanently into retirement.