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By Jonathan Mahler

Mr. Mahler is a staff writer for The New York Times Magazine and the author of the forthcoming book “The Gods of New York: Egotists, Idealists, Opportunists, and the Birth of the Modern City: 1986-1990,” from which this essay is adapted.

It’s been a long time since New York has been this invested in a mayoral race. And it’s been even longer since the rest of America has taken an interest in the city’s politics. But ever since Zohran Mamdani’s stunning victory in the Democratic mayoral primary in June, all eyes have been on New York. Even a certain New York expat with a country to run is mulling whether to insert himself into the race.

Americans are right to pay attention, and more is at stake than they may realize. What’s happening in New York today is not just the political triumph of youth and charisma over age and cynicism. And it’s not just the prospect of a dynamic progressive leader rising up from the smoldering remains of an unpopular Democratic Party. Whatever transpires with Mr. Mamdani’s political career, his emergence signals the end of a momentous chapter in American history.