Mr. Mamdani’s victory came in the face of fierce attacks on his policies and Muslim heritage from business elites, conservative media commentators and Mr. Trump himself.

“If anyone can show a nation betrayed by Donald Trump how to defeat him, it is the city that gave rise to him,” Mr. Mamdani said in a victory speech to supporters.

“In this moment of political darkness, New York will be the light.”

The Democratic party’s victories in the governor’s races in Virginia and New Jersey suggested a shift in political mood as the country looks to next year’s midterm elections when control of Congress will be up for grabs.

In another significant win for Democrats, voters in California approved a proposition to redraw electoral districts in a bid to neutralize gerrymandering efforts ordered by Trump in other States.

Mr. Trump refused to take any blame for Tuesday’s results. In a post on his Truth Social network, he cited anonymous “pollsters” suggesting the Republican defeats were down to the government shutdown and the fact that his own name wasn’t on ballots.

House minority leader Hakeem Jeffries said “Democrats are smoking Donald Trump and Republican extremists throughout the country,” writing on X that “the Democratic Party is back.”

Mr. Mamdani, a State lawmaker for New York’s Queens borough, appealed to voters by pledging to tackle the soaring cost of living, offering free city bus travel, childcare and city-run grocery stores.

He focused on living costs facing ordinary New Yorkers, building support through his informal personal style, social media savvy and a massive canvassing ground game.

“The next and last stop is City Hall,” Mr. Mamdani said in a video posted to X after his victory was declared.

The self-described socialist was virtually unknown before his upset victory to secure the Democratic nomination over former governor Andrew Cuomo who he trounced again Tuesday (November 4).

There was a party atmosphere at his results viewing gathering for supporters being held at a storied concert venue in Brooklyn, with the 34-year-old expected to speak later in the night.

Mr. Trump made an eleventh-hour intervention in the race, calling Mr. Mamdani who will become New York’s first Muslim mayor when he takes office in January, a “Jew hater.”