But if Mr. Sanders is not the nominee? The group passed a resolution Friday afternoon saying that it will not officially endorse anyone else, regardless of what that might mean in a general election against Mr. Trump. For D.S.A., it is “Bernie or Bust.”

The lopsided passage of the resolution, known by that pithy nickname, prompted happy whistles in the ballroom of the Westin Peachtree Plaza, one of Atlanta’s few union hotels. The silent, raised-arm, wiggling-hands of American Sign Language applause, which over the convention’s three days crested throughout the room like ocean waves, was unusually ecstatic.

The vote could not help but flabbergast a certain kind of mainstream liberal, anxious that left-wing purity might detract from unseating President Trump next year. Mr. Sanders himself has said there are “no ifs, buts, or maybes” about the stakes in 2020, pledging in a recent interview, “If I lose this thing, I will be there doing everything I can to defeat Trump.”

Charles Lenchner, a D.S.A. delegate who supports Mr. Sanders but opposed the Bernie-or-Bust resolution, concurred. “Defeating Trump is the most important priority for a progressive or leftist,” he said.