Washington DC - President Donald Trump threatened on Monday to withhold federal funds from New York City if it is led by Zohran Mamdani, in an unprecedented attempt to sway Tuesday's mayoral election.

The night before polls opened, Trump called the 34-year-old candidate a "communist" who would leave the city with "ZERO chance of success, or even survival" on his Truth Social platform.

He said if Mamdani wins, "it is highly unlikely that I will be contributing Federal Funds, other than the very minimum as required."

Polls show Mamdani leading by a wide margin over former governor Andrew Cuomo, who is running as an Independent after being trounced by the young socialist in June's primaries.

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Trump also said voting for Republican candidate Curtis Sliwa would effectively help Mamdani, instead urging a vote for Cuomo.

"Whether you personally like Andrew Cuomo or not, you really have no choice. You must vote for him, and hope he does a fantastic job," he wrote.

Mamdani, a member of the New York State Assembly and affiliated with the left-wing Democratic Socialists of America, has campaigned on policies including rent control and free bus and childcare services, funded by higher taxes on the wealthy and businesses.

Cuomo, who was forced to resign in disgrace amid a wave of sexual harassment accusations, fully welcomed Trump's endorsement, despite previously promising to "stand up" to the far-right president.

If elected, he would become the first Muslim mayor of New York City, which has more than 8 million residents. Polling places close at 9 PM, after which the first results are expected.