Thursday evening will feature another Democratic presidential primary debate, and praise be to a merciful God, this time there is only one night and one group of contestants. Finally all three of the major contenders in what is rapidly coalescing into a three-person race — Joe Biden, Bernie Sanders, and Elizabeth Warren — will confront each other face-to-face.

The major question for the evening will undoubtedly be whether anyone can take Biden down a peg — particularly in his perceived edge in a general election matchup against Trump. Despite a very rough last few weeks of campaigning, he remains far ahead of either Warren or Sanders in most polls (though a few have found both tied or ahead).

It will be a hard sell just on data, as the evidence on so-called "electability" is all over the place. A recent Washington Post/ABC poll found all three of the top Democratic candidates beating Trump handily — Biden by 16 points, Sanders by 12, and Warren by 11. This order generally tracks with previous polls, though with a larger margin. It certainly appears that just about any Democrat ought to have a reasonable chance against Trump.

It is rather odd how Sanders has consistently polled as the second-strongest candidate against Trump, despite being the most left-wing candidate in the race. Indeed, a recent poll of Texas (Texas!) showed him doing best of all, defeating Trump by six points as compared to four for Biden. Now, one should not assume Texas is in the bag, of course, but it certainly provides evidence for the proposition that Sanders' radical politics have not harmed him much in national perceptions.

I would hazard a guess that this comes largely down to class affect. Biden and Sanders have wildly different politics and records, but both project a sort of outsider working-class persona. Warren has quite similar positions to Sanders on most things, but comes across as much more intellectual and professional. Looking at the diverse class and racial background of Sanders supporters, we might reasonably surmise that people with less education and income — people who are usually less committed politically — tend to be attracted to people who don't seem like educated elites.

That group will be an important one in 2020 — both in potentially peeling off some of the former Obama voters who voted for Trump and third parties in 2016, and more importantly, activating some of the huge population of nonvoters.

Still, it sure doesn't appear that Democratic voters are carefully weighing the evidence about who would be best against Trump (much less who would be best equipped to carry out the drastic social overhaul that is desperately needed to head off a more competent Trump successor). Polls and anecdotal reporting describe a Democratic base that is terrified of Trump, and clinging to vague Pundit Brain nostrums about what sort of candidate is likely to win.

But that means that the "electability" sham is almost entirely based on perception — a sort of self-fulfilling prophecy whereby a polling lead drives favorable coverage, which instills an aura of victory and inevitability, which keeps the polls up, and so on.

It is tough to break such a cycle, but if it does happen, Biden's collapse is likely to be swift, because being able to defeat Trump is the entire rationale for his candidacy. He's not a policy maven, he is old and has visibly lost a step mentally, and unlike Sanders (who is even older) his record is very badly out of step with the modern Democratic Party's expressed views.

So one should expect all the candidates to attack Biden at least obliquely. Going ferociously negative might backfire, because such tactics often rebound on the attacker among the polite Democratic base (as Kamala Harris has learned to her chagrin). But someone at least is likely to try to trip him up on his words, or delicately point it out if he commits one of his signature flubs. If he can be made to look hesitant, frail, and confused, that is likely to spark some fear in Biden supporters that he simply wouldn't be able to handle a gruelling campaign and presidency, and lead some to jump ship. And once that decline gets going, it might easily continue.

At any rate, if something of that sort doesn't happen, it's very easy to imagine Biden cruising to the nomination. So far, week after gaffe-filled week has barely dented his support. Somebody better lay a glove on him, and soon.

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