Democratic Primaries

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.....

These voters’ willingness to unite against Mr. Sanders helped Democratic Party leaders stave off his insurgent campaign and has made Mr. Biden the all-but-certain Democratic nominee. The convergence behind Mr. Biden also highlights a critical difference between this year’s primary and what happened to the Republican Party in 2016. Four years ago, establishment Republicans were openly skeptical of Mr. Trump after his victories in early primary states, but a fractured field and split primary vote allowed him to amass an insurmountable delegate lead, reshaping the party in the process.....

Ahead of Mr. Sanders’s presidential run in 2020, his campaign did not concern itself with smoothing tensions among voters who supported Mrs. Clinton in 2016. He did not seek the endorsements of many party leaders, who were always unlikely to back him, but could have been swayed from being openly antagonistic to ambivalent.

As a result, after a strong finish in Iowa and wins in New Hampshire and Nevada, Mr. Sanders did not benefit from an assumed truth of presidential campaigns: that early-state victories help bring in voters from other factions. Instead, people like Lori Boerner of McLean, Va., said Mr. Sanders’s performance sent them searching for a candidate who could stop his rise, and after the South Carolina primary, they landed on Mr. Biden.