It is also a recognition that Mr. Sanders has moved the party to the left, introducing policies like “Medicare for all” and tuition-free public college that are now embroidered into the fabric of the party.

But perhaps, above all, it is a projection of how desperate Democrats are to beat Mr. Trump. Establishment Democrats recognize that if Mr. Biden is to win in November, he will need the backing of Mr. Sanders’s loyal supporters, some of whom have expressed reluctance, if not disgust, at the notion of voting for the former vice president. By showing Mr. Sanders gratitude, they are hoping to mollify his supporters, too.

On Wednesday, Mr. Sanders’s campaign manager, Faiz Shakir, was circumspect about the establishment’s newfound respect for his boss.

“He’s a person who has unrivaled integrity among many politicians,” he said. “When the dust settles, people recognize that integrity.”

Mr. Sanders, who is 78, said this week in an interview with The Associated Press that it was “probably a very fair assumption” that this presidential campaign was his last. But Mr. Shakir has rejected the idea that Mr. Sanders is guided by a wish to cement a positive political legacy in the eyes of a party he has long loathed.

“That’s not the way he thinks,” he said. “He does not think on legacy terms.”

For months, Mr. Sanders had said he would do everything he could to support the eventual Democratic nominee. “I hope it’s me,” he would say, before quickly committing to work with whomever it was to defeat Mr. Trump.