Mr. Sanders pointed on Sunday to Ms. Klobuchar’s and Mr. Buttigieg’s decisions to drop out as the reason he lost several key states to Mr. Biden on Super Tuesday.

“The establishment put a great deal of pressure on Pete Buttigieg, on Amy Klobuchar, who ran really aggressive campaigns,” he told Chuck Todd on NBC’s “Meet the Press.” “If they had not withdrawn from the race before Super Tuesday, which is kind of a surprise to a lot of people, I suspect we would have won in Minnesota, we would have won in Maine, we would have won in Massachusetts.”

For Ms. Harris, the decision to endorse Mr. Biden is particularly striking, because one of the biggest moments of her campaign was a forceful debate-stage exchange with him.

“It was hurtful to hear you talk about the reputations of two United States senators who built their reputations and career on the segregation of race in this country,” she said in June during the first Democratic debate.

“And it was not only that, but you also worked with them to oppose busing,” she added. “And, you know, there was a little girl in California who was part of the second class to integrate her public schools, and she was bused to school every day. And that little girl was me.”

The exchange gave her a lift in the polls and a temporary burst of momentum that she was not able to sustain. At a fund-raiser on Friday, Mr. Biden’s wife, Jill Biden, described Ms. Harris’s attack at the debate as a “punch to the gut.”

Some of Ms. Harris’s advisers were privately frustrated that she endorsed Mr. Biden after California voted on Super Tuesday and when it was already clear that he had become the front-runner. Some were hoping in January that she would rally to Mr. Biden at a point when her endorsement would have been more valuable — and more politically daring.