Mr. Mamdani nonetheless chose to share the spotlight with Ms. James, who was indicted last week on federal charges of bank fraud and making a false statement after warring with Mr. Trump. Her support of Mr. Mamdani — and the platform she commanded on Monday night — took on extra significance. In her first public appearance since the indictment, Ms. James spoke defiantly against the charges, raising her fist in the air before vowing to fight them.

“You come for me, you’ve got to come for all of us, all of us, every single one of us,” she said, her voice rising. “We’re all in this together.”