The Sanders campaign also circulated a memo to surrogates and supporters highlighting “the contrasts that we expect to now be in the spotlight as we head into Super Tuesday and beyond.”

“We are now entering the phase of the primary in which the differences between Bernie and Biden will take center stage,” the memo said. “These differences make clear that the choice between these two candidates is stark.”

Since Mr. Sanders got into the race a year ago, his top advisers have ached for a two-way race with Mr. Biden, viewing the moderate former vice president as the perfect foil for Mr. Sanders’s promise of a political revolution: a throwback Democrat who touts his ability to work with Republicans and who has been criticized by rivals for telling donors that under a Biden administration, “nothing would fundamentally change.”

But the reality of such a direct confrontation with Mr. Biden, a five-decade fixture in Washington politics, poses challenges for Mr. Sanders, whose style of grass-roots politics and long history as an outsider means he cannot muster the same kind of institutional forces. Instead, he relies on a loyal army of individual donors who give $18 at a time, and a progressive network that for all of its ambition remains in some ways disjointed and uncoordinated.

“There’s a reason the establishment has power, keeps power and maintains powers, because these are the things that they do well,” Mr. Shakir said.

Complicating matters, Mr. Sanders’s chief ideological rival, Senator Elizabeth Warren of Massachusetts, remains in the race despite finishing out of the top two in all of the first four nominating contests — including a distant fifth in South Carolina. Though there have been fissures in the united facade their two campaigns presented for much of the primary race, Sanders aides respect her and do not want to publicly pressure her to drop out.

“We respect the fact that she’s going to make whatever decision she makes, and she should be allowed to do that,” Mr. Shakir said, adding that she “should be given the time and space” to determine her path forward.