The former vice president has won in the South, in the Northeast, in the Midwest. He has won large states and small ones. He has won in places where his strength with African-Americans could carry him and in others where such residents are fewer and far between.

So thorough is Mr. Biden’s hold on the party now that any collapse would probably require a political U-turn as sharp as the one that precipitated his rise. And as 2020 lurches into a logistically uncertain phase, with rallies canceled over coronavirus fears and voters more likely to privilege steady leadership, Mr. Biden is consistently and significantly outpacing Mr. Sanders on the crucial measure of whom to trust in a crisis, according to exit polls…

The result — implausible as it seemed last month, when Mr. Biden faltered so badly in Iowa and New Hampshire that establishment Democrats indulged fantasies about a contested convention to stop Mr. Sanders — is a race that could be nearing its functional end.