And perhaps accordingly, Mr. Weaver has been cast as the heavy in news reports that the dwindling Sanders campaign is riven between die-hards and professional consultants who prefer to get back on the good side of the Democratic establishment. Mr. Weaver disagreed.

“The senator, he drives this train,” Mr. Weaver said. “It’s not a staff-driven or consultant-driven campaign.”

Mr. Weaver has made enemies on Capitol Hill and inside the Democratic National Committee, but he appears to get along quite well with his opposite number in the Clinton campaign, Robby Mook. In fact, Mr. Mook and Mr. Weaver seem to have the kind of back-channel entente — and camaraderie — that the Soviet foreign minister Eduard Shevardnadze and Secretary of State Jim Baker shared during the Cold War.

“I get along with Jeff extraordinarily well,” Mr. Mook said, noting that he speaks to Mr. Weaver by phone often. “He is obviously a tough opponent who did a masterful job, but I consider him a good and close colleague.”

Tad Devine, Mr. Sanders’s chief strategist, is described by some political analysts as the insider most eager to make peace with the Clinton forces. Mr. Devine said he thinks of Mr. Weaver as a friend and ally, not a rival. “Jeff looks tough and aggressive on television,” Mr. Devine said. “He’s smart and he’s not afraid to stay stuff. But off camera, he’s steady, really smart and good company. We laugh a lot.”

It was not so jolly inside the Sanders war room on Hollywood Boulevard in Los Angeles on the night of the California primary. The two top aides sat near each other as the returns began coming in, not so much together as in parallel play. Mr. Devine watched CNN on a laptop and made droll comments about the commentators. Mr. Weaver was crouched over a different computer with the campaign’s pollster Ben Tulchin, staring at early returns.