Those pulling for the surprising primary winner include some inside the business community and even Wall Street—they are just quiet.

Mr. Mamdani’s victory upended city politics and reverberated nationally. He relied on a memorable message, charisma and a strong ground game.

New York State Assembly member Zohran Mamdani was way ahead after ballots were counted last week, but the city’s ranked-choice system meant he could not clinch the nomination until second-choice votes were counted Tuesday.