It is now almost certain that he will have a far more difficult path to the nomination. Democrats are overwhelmingly focused on finding a candidate they believe can defeat Mr. Trump, having made clear in surveys that ousting a president they loathe is their top priority. Now, in addition to questions about whether he is too far to the left to win over general election voters, Mr. Sanders will have to face doubts about his physical capabilities.

Further, unlike in 2016, progressive voters have an alternative to Mr. Sanders in this race. While some of his bedrock supporters will not abandon him, other left-wing voters will gladly cast a vote for Ms. Warren. That was not the case when Mr. Sanders faced Mrs. Clinton in the last primary.

Mr. Sanders returned to Burlington over the weekend, after spending three days in a hospital in Las Vegas. His campaign said he felt chest pains during an event last Tuesday, and he was taken to the hospital, where two stents were inserted into an artery.

Since then, his campaign has insisted that Mr. Sanders does not intend to drop out of the race. During a telephone call with staff members on Monday, Mr. Sanders said he felt “more strongly about the need for a political revolution today than I did when I began this campaign.”

Jane Sanders, who remains one of Mr. Sanders’s closest advisers, said that idea was “something that the entire campaign, and especially me, have been saying for months — not for his health but for the ability to keep up that kind of a pace for everybody else, too.”

Campaign officials also downplayed Mr. Sanders’s remarks.

“As Bernie said, we are going to have an active campaign,” Faiz Shakir, Mr. Sanders’s campaign manager, said in a statement on Tuesday evening. “Instead of a breakneck series of events that lap the field, we are going to keep a marathoner’s pace that still manages to outrun everyone else.”

Another adviser, who spoke on the condition of anonymity to discuss a sensitive topic, said Mr. Sanders had been thinking about changing the pace of his campaign schedule for some time, and aides discussed this with him when he was in the hospital.