NEW YORK — Zohran Mamdani was still asleep early in the morning after June’s Democratic primary when the phone calls started flooding in.

A young democratic socialist, Mamdani had just toppled former Gov. Andrew Cuomo, upending New York’s power structure in an upset so stunning and so swift that even he had not fully seen it coming.

Now, titans of the city establishment were clogging up the phones of the candidate and his small team. Most did not sound happy.

“It’s a great day in New York,” Morris Katz, Mamdani’s 26-year-old political adviser, told real estate magnate William C. Rudin in one of the conversations.

The businessman paused. That’s certainly not how I see it, he replied.

It took just hours to become clear that the power brokers and civic gatekeepers accustomed to running N

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