For several weeks, Mr. Sanders surged. In Iowa, he finished in a virtual tie for first with Pete Buttigieg, the former mayor of South Bend, Ind., who, at 38, was less than half his age. He then narrowly beat Mr. Buttigieg in New Hampshire, before dominating the field less than two weeks later in the Nevada caucuses.
Suddenly there was talk that Mr. Sanders would run away with the nomination. But his loss in South Carolina to Mr. Biden, who had emerged as the leading moderate in the race, brought his momentum to an abrupt halt, raising significant questions about his weaknesses with black voters and an overriding failure to expand his base.
Three days later, as centrist Democrats rushed to support Mr. Biden, Mr. Sanders lost all of the Southern states voting on Super Tuesday as well as states like Massachusetts and Minnesota that he had expected to win.
Mr. Sanders quickly canceled a planned rally in Mississippi and deployed resources to Michigan, a tacit acknowledgment that the Midwestern state had become something of a must-win for his candidacy to survive.
But it was not to be. The momentum had turned solidly to Mr. Biden, who won Michigan easily then a week later captured three big delegate prizes — Florida, Illinois and Arizona.
By then, the coronavirus was disrupting the rest of the political calendar, forcing states to postpone their primaries until June. Mr. Sanders has spent much of the intervening time at his home in Burlington without his top advisers, assessing the future of his campaign. Some close to him had speculated he might stay in the race to continue to amass delegates as leverage against Mr. Biden.
But in the days leading up to his withdrawal from the race, aides had come to believe that it was time to end the campaign. Some of Mr. Sanders’s closest advisers began mapping out the financial and political considerations for him and what scenarios would give him the maximum amount of leverage for his policy proposals, and some concluded that it may be more beneficial for him to suspend his campaign.