In Michigan, where Mr. Sanders had focused much of his attention in the last week, he was leading in none of the state’s 83 counties as of early Wednesday, and trailing Mr. Biden by double digits statewide. In Missouri, Mr. Sanders was also trailing in every single county in a state where he won dozens of counties in 2016. And in Mississippi, Mr. Sanders was getting thrashed, with Mr. Biden topping 80 percent of the vote.

The margin of black support for Mr. Biden over Mr. Sanders in Mississippi was breathtaking: 87 percent to 10 percent, according to the exit poll.

The calendar does not get significantly easier for Mr. Sanders in the next two weeks. Three of the four states that vote next week have sizable black voting populations: Illinois (a 28 percent black electorate in the 2016 primary, according to the exit poll), Ohio (20 percent) and Florida (27 percent).

Given Mr. Sanders’s struggles in Michigan on Tuesday, it is not clear why he would do significantly better in nearby Ohio and Illinois. Then the next week comes Georgia, where black voters made up half of the Democratic electorate four years ago.

But it was hardly just black voters lifting Mr. Biden. In Missouri, for instance, the story was about white voters who sided with Mr. Sanders in 2016 but backed Mr. Biden in 2020. Exit polls showed that Mr. Sanders led Mrs. Clinton by nine percentage points among white voters there; Mr. Biden was up by 12 points this year.