But what if Mr. Trump is wrong? What if every elected official hustling to endorse Mr. Biden, after long resisting, is wrong, too? Recent election history, especially Mr. Trump’s, has been unkind to conventional wisdom.
Only a couple of weeks ago, some Biden allies were talking quietly about how he could, at least, end his campaign with dignity: Hang on narrowly in South Carolina, hopefully, and bow out, statesmanlike, if Super Tuesday went sideways as many expected.
That Mr. Biden’s fortunes have changed says more about the context of this primary than the content of his campaign. Current and former competitors, including Ms. Warren, former Mayor Pete Buttigieg and Senator Amy Klobuchar, all strained to connect with black and Latino voters. Mr. Bloomberg flailed on the debate stage as he stepped out from behind the reputational curtain of his ubiquitous advertisements. And many in the party have remained uncomfortable with the kind of unswerving progressivism that Mr. Sanders demands.
The depth of this last concern with Mr. Sanders is uncertain. Entering Tuesday, his predictions of runaway progressive turnout to his cause, a central premise of his case for his own electability, had not necessarily come to pass.
It is also quite possible, after all the Tuesday states are accounted for, that the delegate picture will look very competitive, particularly as full tallies from California roll in. Mr. Sanders remains popular across much of the party, even among many who did not consider him their first choice.
If nothing else, a Biden-Sanders matchup is the logical venue for the party’s foremost ideological debate about the proper scope and ambition of government — about whether Mr. Trump is a symptom of longstanding national ills or an “anomaly,” as Mr. Biden has suggested, whose removal should be the party’s chief animating priority.