Last summer, Mr. Sanders dined with another group of House members to make the case that his economic message and outsider’s approach would have wide traction in the general election. “Bernie’s argument is, ‘Don’t pigeonhole me as just a progressive candidate,’” said Representative Dan Kildee of Michigan, who attended the dinner.
But Mr. Sanders has done little over the last month to reassure skeptics within his party, striking a posture of defiance that may have helped accelerate the late movement toward Mr. Biden. Most senior Democrats are hoping that Mr. Sanders’s forward march can be slowed, and some have urged Mr. Obama to intervene.
Mr. Obama has shown no inclination to do so, reasoning that he must hang back to preserve his ability to help unify the party at the end of a messy nominating process.
Mr. Biden’s aides see that argument as so much malarkey, and they have conveyed as much to Mr. Obama’s inner circle, arguing that the former president’s leverage would evaporate once Mr. Sanders accrued millions of votes. They have pleaded with Mr. Obama’s camp to distance him from Mr. Bloomberg, who has run saturation-level advertising showing images of himself with the former president. When a poll came out last week showing that many Democratic voters believe Mr. Obama is supporting Mr. Bloomberg, the Biden campaign shared it with Mr. Obama’s aides.
Party leaders are already preparing for the possibility of a contested convention, and many superdelegates have indicated they are open to selecting a nominee besides Mr. Sanders if he collects the most delegates but falls short of a majority.
Tom Perez, the D.N.C. chairman, has been clear in conversations with allies that he will abide by the current rules and would not resist an effort by superdelegates to determine the nomination on a second ballot. Referring to the number of delegates needed for a majority, Mr. Perez told one Democrat, “1991 delegates are required,” and noted that Mr. Sanders is familiar with the guidelines.
With the identity of the Democratic standard-bearer unknown, Mr. Schumer has begun to ponder ways of uniting the party behind whoever that person might be: The Senate leader is particularly focused on the idea of nominating an African-American woman for vice president, mulling names like Senator Kamala Harris of California, Stacey Abrams of Georgia and Representative Val Demings of Florida, according to people who have spoken with Mr. Schumer.