But he has always been disdainful of the art of politics and had to be nudged into wooing even friendly Democratic leaders. As Ms. Warren relentlessly courted Ms. Ocasio-Cortez last fall, Ms. Ocasio-Cortez’s advisers had to prod Mr. Sanders’s aides into having him call her — a conversation that eventually led to her endorsing him.

Pushing Mr. Sanders to reach out to “establishment Democrats” whom he regularly taunted was even tougher — despite the best efforts of even some of his staunchest supporters on the left. Ms. Ocasio-Cortez repeatedly urged the campaign to broaden Mr. Sanders’s message and seek out new allies, outside his familiar base. (In a statement, Ms. Ocasio-Cortez denied any “tension or major disagreements” with Mr. Sanders.)

RoseAnn DeMoro, the former leader of the nurses union who was one of Mr. Sanders’s most ferocious surrogates in 2016, and the actor John Cusack, another ally, both pressed the campaign to refocus Mr. Sanders’s pitch on a general-election audience, people familiar with their entreaties said.

Mr. Cusack urged the Sanders campaign to address voters beyond its progressive base, proposing that the senator give a speech at St. Francis College in Brooklyn and citing its namesake’s connection to environmentalism and fighting poverty. “His campaign needs to create a unit that is charged with outreach to groups who do NOT identify as progressive, but have strong views that are aligned with his. EXPAND EXPAND EXPAND” Mr. Cusack wrote to top Sanders allies and advisers, in an email obtained by The New York Times.

Mr. Sanders was not interested in moving in that direction. Some advisers, who endured the divisive 2016 campaign, believed that it was only after seizing a dominant advantage that Mr. Sanders could attempt to make peace with a Democratic establishment that remained intensely wary of him.

Arriving in Charleston, S.C., ahead of the Feb. 29 state primary, Mr. Weaver said the campaign had not yet sought a working relationship with figures like the House Speaker Nancy Pelosi because they wanted first to demonstrate the full sweep of their coalition on Super Tuesday three days later. He reached for a Civil War analogy to explain the muscle-flexing strategy. Abraham Lincoln did not issue the Emancipation Proclamation, Mr. Weaver said, until after Union troops turned back the Confederacy at the bloody battle of Antietam.