New Yorkers elected young leftist Zohran Mamdani as their next mayor Tuesday and Democrats won two key state governor races as US voters delivered an early warning to President Donald Trump ahead of 2026's midterms.

The clean sweep of the top contests among several ballots nationwide will boost morale for Democrats bruised by Trump's onslaught since returning to the White House, and set alarm bells ringing in Republican circles.

Mamdani's victory came in the face of fierce attacks on his policies and Muslim heritage from President Donald Trump, business elites and conservative media.

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.

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."

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," Mamdani said in a video posted to X showing the doors of a subway train opening to City Hall station.

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.

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.

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

Republican candidate Curtis Sliwa, founder of the Guardian Angels citizen crime patrol group, came in third after weeks of Cuomo insisting he bow out to increase his chances.

"It's typical of Andrew, he's always blaming other people for his own failures," Sliwa told AFP ahead of the result. "Of course he's going to blame me."

Prominent business people including Bill Ackman noisily attacked Mamdani and funneled cash to his rivals, while conservative media including The New York Post published blanket negative coverage.

Turnout was high in this year's vote with 1.45 million people casting ballots by 3 pm (2000 GMT) -- more than the total number of voters in the 2021 race.

Uphill struggle

Mamdani's improbable rise highlights the Democratic Party's debate over a centrist or a leftist future, with some leading national figures offering only tepid endorsements of Mamdani ahead of voting.

Cuomo said there was "a civil war in the Democratic Party." 

"You have an extreme radical left that is run by the socialists that is challenging what they would call moderate Democrats. I'm a moderate Democrat," he said after voting.

Syracuse University political science professor Grant Reeher said ahead of the result mayor Mamdani would face an uphill battle "at the center of all of these nasty political controversies." 

"Everybody's got their knives out, and it's a very difficult city to govern," he told AFP.

There was a grim mood among some attendees at Cuomo's results party with some attendees predicting Trump would immediately deploy the National Guard to the city.

Others blamed Sliwa for splitting the center-right vote.

In New Jersey, Democratic Party candidate Mikie Sherrill, a former Navy helicopter pilot, beat out Republican Jack Ciattarelli, a businessman backed by Trump.

In Virginia's race for governor, Democratic candidate Abigail Spanberger unseated a Republican incumbent to beat Virginia's Lieutenant Governor Winsome Earle-Sears.

Both sides wheeled out big guns, with former president Barack Obama rallying support for Spanberger and Sherrill.

"We've still got plenty of work to do, but the future looks a little bit brighter," Obama said responding to the wins.