Just weeks ago, aides and supporters of Mr. Sanders had optimistically pointed to the Wisconsin primary as a possible springboard for revitalizing his campaign, viewing him as well positioned to recapture a state that he dominated in his 2016 presidential bid — even as he continued to lose race after race to his rival, former Vice President Joseph R. Biden Jr.

It may instead be his last stand. A recent poll showed him far behind Mr. Biden in the state, and top Sanders advisers have all but ceased speaking about his chances there. Results are not expected to be known until possibly next week, but if Mr. Sanders suffers another big loss, it is certain to intensify the growing calls for him to exit the race.

In the weeks since the coronavirus crisis put the campaign on hold, Mr. Sanders has tried to convert his political operation into a virus-focused initiative. The effort has kept his supporters engaged, applied political pressure to protect workers and raised millions of dollars for charities to help people dealing with the outbreak. But it has done little to shift the dynamics of his contest with Mr. Biden.