So when it became clear that the Senate seat held by Hillary Rodham Clinton would become open, Mr. Cuomo restrained himself from overt campaigning and retreated to the background. That left the stage to Ms. Kennedy, who has marched out front and become the candidate everyone is talking about — and the favorite for the appointment.

That, friends say, has left Mr. Cuomo feeling outfoxed and frustrated. “It’s driving him crazy,” said one confidant of Mr. Cuomo’s, who spoke to the attorney general about the Senate seat this week. “He’s boxed in. He can’t do anything except fume, and he is fuming.”