“It’s difficult to compare tonight to four years ago: There is an incumbent president, who Democrats are just focused on replacing, and that was not the case four years ago,” Mr. Brewer said. “Sanders was the underdog four years ago, and frankly Hillary Clinton took the state for granted and did not campaign here vigorously, and so forth. It’s really apples and oranges.”

Perhaps nothing better illustrated the direction of this nominating contest, though, than the surge in voter turnout. A centerpiece of Mr. Sanders’s campaign has been his promise to win by inspiring a wave of new voters in the primary race and the general election.

But for the second consecutive week it was Mr. Biden who was the beneficiary of an increase in turnout compared with that of the 2016 race, often in the sort of suburban jurisdictions that powered the Democrats’ big gains in the midterm elections. In Oakland County, Mich., the Detroit suburb where Representative Haley Stevens picked up a Republican-held seat in 2018, Mr. Biden was winning by nearly 30 points and turnout was on track to be about twice what it was four years ago.

The next few rounds of primaries are focused on big, diverse states where Mr. Biden is seen as having a solid advantage, including Florida and Georgia. And a number of states where Mr. Sanders won caucuses four years ago have since switched to hold primary elections, which tend to be less controlled by ideological activists, leaving Mr. Sanders without an obvious place to make a comeback.

The contests on Tuesday included four states that Mr. Sanders carried against Mrs. Clinton in 2016: Michigan, Idaho, North Dakota and Washington.

But there has been no indication that many Democrats have been having second thoughts about embracing Mr. Biden, to whom the party’s voters turned abruptly last week as a safe option after a chaotic and confusing primary season. With the race effectively down to just Mr. Biden and Mr. Sanders, many Democratic leaders have clambered aboard the former vice president’s campaign.

After Mr. Biden carried South Carolina on Feb. 29, rivals like former Mayor Pete Buttigieg of South Bend, Ind., and Senator Amy Klobuchar of Minnesota quickly dropped out of the race and endorsed him. Once the Super Tuesday results were known, Michael R. Bloomberg, the former mayor of New York City, did the same. By contrast, Mr. Sanders has not managed to unify support even on the left; Ms. Warren, a fellow progressive, has declined to issue an endorsement since leaving the race last week.