A Democratic Socialist state assemblyman, Zohran Mamdani, is leading in the primary with 43.5 percent of the vote and is on track to be the Democratic nominee for New York City mayor.

Mr. Mamdani did not reach the 50 percent threshold to win the primary in the first round of ranked-choice voting, so no winner will be declared until July 1 at the earliest, after all the mail-in ballots have been validated and counted. Yet Mr. Mamdani far exceeded all polls — including the Emerson College poll released this week that was the first major nonpartisan poll to show him winning — and he is expected to gain the largest share of votes from the third-place candidate, Brad Lander, leading him to victory.

This is a major upset for a former New York governor, Andrew Cuomo, who earned 36.4 percent of the vote. He was trying to make a political comeback after resigning from the governorship in 2021 in disgrace, facing sexual harassment allegations from 13 women. He led in the polls since entering the race in March — until this week.

Mr. Cuomo conceded Tuesday night, saying he called Mr. Mamdani to congratulate him. “Tonight was not our night,” Mr. Cuomo said. “Tonight was his night. He deserved it. He won.”

The race was largely seen as a battle over the future direction of the Democratic Party. Mr. Cuomo, 67, pitched himself as a moderate with the managerial experience to turn the city around from crisis and stand up to President Trump. Mr. Mamdani embraced the socialist label.

Mr. Cuomo led a cautious campaign, sticking to controlled events at Black churches and with unions. Plagued by sexual harassment accusations and the Covid nursing home scandal, he rarely took press questions. He drove to campaign events instead of taking the subway. His polling numbers stagnated in the high 30s. That’s what he earned Tuesday night.

Mr. Mamdani, by contrast, entered the race with no name ID but adeptly ran a high-energy insurgent campaign, utilizing social media to build a coalition of far-left young New Yorkers. Instead of running from his membership in the Democratic Socialists of America, Mr. Mamdani embraced it and promised, if elected mayor, to make buses and childcare free and to freeze rents on stabilized apartments.

A Muslim born in Uganda and raised in New York City by a movie director mother and Columbia professor father, Mr. Mamdani fashioned himself a defender of the working class — even though he and his base are white and Asian college-educated New Yorkers in Manhattan and Brooklyn. He got into hot water for his views on Israel, which he tried to moderate in the last week, though he refuses to condemn the phrase “globalize the intifada.”

His message and social media stardom helped earn Mr. Mamdani endorsements from other progressive superstars such as Congresswoman Alexandria Ocasio Cortez and Senator Sanders. A fashion model and influencer, Emily Ratajkowski, posted a video with Mr. Mamdani while wearing a “Hot Girls for Zohran Mamdani” T-shirt on Tuesday morning to her 29 million followers on Instagram. Even an ousted Democratic National Committee vice chairman, David Hogg, posted a video to get out the vote for Mr. Mamdani.

“This election will be determinative of the future of the Democratic Party,” a Democratic strategist, Hank Sheinkopf, tells the Sun. He also says it’s a sign of the demographic shifts happening in the city. The old Democratic base of white, working-class ethnics and Blacks is declining in numbers, while the number of Muslim Americans and Asians is rising. “If Mamdani wins, it will also be a reflection of the end of the influence of Jews in New York City,” he says.

Mr. Mamdani also used the ranked-choice voting system to his advantage. He and Mr. Lander cross-endorsed each other last week, imploring their voters to rank the other second. Mr. Cuomo told reporters he only voted for himself.

The winner of the Democratic mayoral primary is usually a shoe-in to Gracie Mansion, but not this year. Mr. Mamdani will likely run as a Democrat and on the Working Families Party line. 

Mayor Adams, a Democrat, did not run in his party’s primary but is instead running on two independent lines in the general election. The Guardian Angels founder, Curtis Sliwa, is running on the Republican line and on an independent line he created. A former United States attorney, Jim Walden, is running as an independent.

It’s unclear whether Mr. Cuomo will run in the general election on the independent ballot line he created, “Fight and Deliver.” His language Tuesday night was vague about his future. When asked Tuesday morning whether he will run as an independent if he loses the primary, Mr. Cuomo responded, “I’m on the general election ballot.”

“New York City used to be the city where, once you win the Democratic primary, it was over,” a political strategist who’s worked on both sides of the aisle, E. O’Brien Murray, tells the Sun. “But that is not what’s happening this year, and anybody can win in November.”