The gravity of this question was apparent Wednesday in Mr. Sanders’s announcement that he would be suspending his campaign. “As I see a crisis gripping the nation, exacerbated by a president unwilling or unable to provide any credible leadership and the work that needs to be done to protect others, I cannot in good conscience continue a campaign that I cannot win,” Mr. Sanders said during a Wednesday morning conference call with his staff.

Embedded in the statement was an acknowledgment that the events of recent weeks had turned the 2020 primary race into less of an ideological contest. The “movement” that Mr. Sanders would frequently liken his campaign to had been rejected by Democratic voters and, then, usurped by events.

In early March, after Covid-19 had grounded him back in Vermont, Mr. Sanders would become less direct in his critique of Mr. Biden and more so of Mr. Trump. He adopted an increasingly disdainful tone against the president. “The first thing we have to do, whether or not I’m president, is to shut this president up right now,” Mr. Sanders said last month in what would be the final Democratic debate of the 2020 primary race. “It is unacceptable for him to be blabbering with un-factual information that is confusing to the general public.”

In a sense, the clear and present threat of the coronavirus softened the debate that had raged for much of 2019 and early 2020. “This virus thing makes it very hard to be political,” said Harry Reid, the former Democratic Senate majority leader. It has focused attention on Mr. Trump in a way that highlights the president’s shortcomings, Mr. Reid added. It also served to minimize the policy differences that were apparent during the Democratic primary race.

“That divide is literally now a matter of life and death,” said Representative Ro Khanna, Democrat of California and a national co-chairman of the Sanders campaign. Mr. Khanna said he believed progressives would eventually win the day and would become the dominant force inside the Democratic Party. Not yet, though.

“Maybe that moment will be in 2024, maybe that moment will be in 2028,” Mr. Khanna said. “But the choice in this election is about as clear as can be right now.”

For much of the primary race, the challenge of Democratic unity had been treated as something that could be put off; or at the very least, as a nuisance that would resolve itself through shared aversion to Mr. Trump. But there had also been signs that the split between rank-and-file supporters of Mr. Sanders’s and any eventual nominee not named “Bernie” might be too wide to fully reconcile.