But for all his undeniable success on Saturday, Mr. Biden remains hamstrung on two fronts: by several competitors still disinclined to stand down, even if they siphon crucial non-Sanders votes, and by a campaign organization that party officials have described as jarringly thin in many of the states that will vote on Tuesday.

His ability to address either of these problems in time is far from assured, in a primary that has already exposed his capacity for bungling on the campaign trail, his enthusiasm deficit with many younger voters and his struggles in fund-raising.

Why would Michael R. Bloomberg step aside, before the states he is contesting have even voted, with hundreds of millions of dollars already sunk into his bid and a conviction that he is the man for the job?

What do Pete Buttigieg and Amy Klobuchar owe Mr. Biden, whom they each dusted in New Hampshire (and, in Mr. Buttigieg’s case, in Iowa), where the voters who spend the most time with their candidates decided that the former vice president could not be trusted to defeat President Trump?