“I didn’t want Bernie to beat Elizabeth in her own backyard. But then, it became much more complicated than that,” said Ms. King, 70. “Are we going to have a nominee who could take on the Republican Party? We have to stop Bernie.”

Rarely has political momentum flipped as quickly as it did in the first half of March, as Mr. Sanders lost serious ground to Mr. Biden before the coronavirus slowed their race. There are well-known reasons for the shift: Moderate candidates like Mr. Buttigieg and Senator Amy Klobuchar of Minnesota rallied around Mr. Biden. He enjoyed demographic advantages, particularly with black voters. And turnout among young voters and liberal nonvoters did not surge, failing to reshape the electorate as Mr. Sanders had hoped.

But beyond ideology, race and turnout, a chief reason for Mr. Biden’s success has little to do with his candidacy. He became a vehicle for Democrats like Ms. King who were supporting other candidates but found the prospect of Mr. Sanders and his calls for political revolution so distasteful that they put aside misgivings about Mr. Biden and backed him instead.

In phone interviews, dozens of Democrats, mostly aged 50 and over, who live in key March primary states like Massachusetts, Virginia, Michigan and Florida, said that Mr. Biden’s appeal went beyond his case for beating President Trump. It was his chances of overtaking Mr. Sanders, the only candidate in the vast Democratic field they found objectionable for reasons personal and political.

For some, like Amy Siegel of Natick, Mass., the anti-Sanders feeling relates back to the 2016 Democratic primary, when she supported Hillary Clinton and believed Mr. Sanders ran a divisive campaign that wounded her ahead of the general election. This time around, Ms. Siegel, 57, initially supported Mr. Buttigieg. But she voted for Mr. Biden in her state’s primary, held on Super Tuesday, days after Mr. Buttigieg dropped out and endorsed the former vice president. Ms. Siegel said she decided to flip her vote even before Mr. Buttigieg exited the race.