Of all the democratic socialists who piled into a Manhattan church on Wednesday evening, none had the cachet of the man handed a microphone toward the meeting’s close.

Mayor-elect Zohran Mamdani offered some pleasantries — “Hello friends, Zohran, he/him, Queens D.S.A.” — before launching into his mission: torpedoing the candidacy of a left-leaning ally, Councilman Chi Ossé, who is attempting to unseat Representative Hakeem Jeffries, the top House Democrat.

The remarkable scene was both a reflection of the tricky political calculuses Mr. Mamdani confronts as he prepares to take office next year and the egalitarian nature of a group that served as the grass-roots organizing machine of his political success.