Unlike in the first nominating contests, when these failures did not stop Mr. Sanders from winning, his inability to expand the electorate was crucial to his losses on Super Tuesday, when Mr. Biden beat him in 10 of 14 states, including Texas, Virginia and North Carolina.

After Mr. Sanders’s dominant performance in Nevada less than two weeks ago, his aides and advisers had operated with new swagger, cautiously optimistic about his chances even in Southern states like South Carolina that they had expected to lose. He competed especially heavily in Virginia, North Carolina, Massachusetts and Minnesota, holding big rallies that doubled as shows of force.

But as the clock ticked toward midnight on Tuesday and his losses began to pile up, the campaign’s buoyant tone shifted sharply. When Mr. Sanders took the stage at an election-night rally in Essex Junction, Vt., he was defiant, declaring that he would “win the Democratic nomination” despite a series of weaker-than-expected results. Aides and advisers insisted it was still early, and urged reporters not to jump to conclusions.

“If you turn off your television at 10 p.m. tonight, you will wake up tomorrow to a different race,” Mike Casca, a spokesman for Mr. Sanders, said at one point.

But by Wednesday morning, that optimism was gone.

“Of course I’m disappointed,” Mr. Sanders said at the news conference in Burlington. “I would like to win every state by a landslide. It’s not going to happen.”

He blamed his underperformance in part on the “venom” of the “corporate media.”

As he has begun to do in recent days, he also framed the race as one between him and Mr. Biden, and drew explicit contrasts between his record and the former vice president’s.

“Joe and I have a very different voting record,” he said. “Joe and I have a very different vision for the future of this country. And Joe and I are running very different campaigns.”