Mr. Biden was carrying both moderate and “somewhat liberal” voters by significant margins. He was winning over women, especially college-educated white women. And he was crushing Mr. Sanders among older voters. And in a consequential shift, Mr. Biden was winning white voters without college degrees, who rejected Mrs. Clinton in 2016, not only in the primary but later in the general election.

Mr. Sanders has some demographic pockets of support. He continues to carry voters under 30 by wide margins, even in states he is losing. He was often leading among all voters under 45 years old.

But older voters were simply turning out in greater numbers.

In Missouri, voters under 45 years old were 41 percent of the electorate in 2016; those same voters accounted for only 30 percent of the electorate in 2020. The biggest jump, in terms of share of the electorate, came among Mr. Biden’s strongest group: those 65 or older, who were 35 percent of the electorate, up from only 22 percent four years ago.

Where Clinton struggled, Biden is dominating.

In many ways, the Michigan primary in 2016 was just like the Michigan primary in 2020: Mr. Sanders versus an establishment Democrat. But the results could not have been more different.

In 2016, Mr. Sanders won the state narrowly over Mrs. Clinton, 49.8 percent to 48.3 percent. His victory was driven largely by white, rural voters: He won almost every county outside the Detroit metropolitan area.

But this time, with a little over half the votes counted, Mr. Sanders has only about 39 percent to Mr. Biden’s 53 percent. And if you want to know why he lost so much ground, look to the same voters who lifted him four years ago.

Mr. Biden is winning handily in the rural counties Mr. Sanders carried last time. In Luce County, on the Upper Peninsula, Mr. Sanders went from 62 percent of the vote in 2016 to 33 percent in 2020. In Grand Traverse County, he went from 65 percent to 42 percent.