In recent days, both Mr. Biden and Senator Elizabeth Warren of Massachusetts said that Mr. Sanders should not become the nominee if he arrived at the convention short of a delegate majority. “Bernie had a big hand in writing these rules,” Ms. Warren said during a CNN forum on Wednesday night. “I don’t see how he thinks he gets to change them now that he thinks there’s an advantage for him.”

Slightly less than 3 percent of delegates have been allocated in the race so far, and Mr. Sanders, of course, can win a majority, making him the nominee. But while Mr. Sanders has demonstrated momentum in the race, winning the most votes in each of the first three contests, he has yet to show that he can expand his coalition enough to set his campaign on a path to capturing the majority of delegates. As a result, some within Mr. Sanders’s own campaign foresee a possible brokered convention.

The argument of Mr. Sanders and his allies — that a plurality of delegates should be sufficient to clinch the nomination — is a different standard than the one laid out in party rules that his team helped draft two years ago. It’s also a reversal of their stance in 2016, when Mr. Sanders encouraged superdelegates to support him over Mrs. Clinton, who secured the majority of pledged delegates.

“The will of the people should prevail,” he said when asked during last week’s debate if the candidate with the most pledged delegates should be the Democratic nominee. “The person who has the most votes should become the nominee.”

Supporters of Mr. Sanders said that blocking him from the nomination if he had the most delegates would repel progressives, and would deliver a second term to Mr. Trump.