“… And so it begins,” President Trump posted to Truth Social on Tuesday night, as the new mayor-elect of New York City, Zohran Mamdani, took to the stage to give his victory speech at the Paramount Theater in downtown Brooklyn.

Mr. Trump may be the master of the political troll, but Mr. Mamdani proved Tuesday night that he can give as good as he gets. The 34-year-old democratic socialist, who won with just over 50 percent of the vote, peppered his speech with attacks not only on Mr. Trump but also on his main challenger, Governor Andrew Cuomo, and on the “big money and small ideas” of the Democratic establishment.

Mr. Mamdani framed his win as “a mandate.” His speech harkened to President Obama with refrains about “hope,” but it was more Trumpian in its populist, unapologetic rhetoric and retributive attacks.

“Donald Trump, since I know you’re watching, I have four words for you: Turn the volume up,” Mr. Mamdani said, looking into the cameras. “New York will remain a city of immigrants, a city built by immigrants, powered by immigrants, and, as of tonight, led by an immigrant.”

The crowd erupted in cheers. This may not bode well for New York City — and that’s putting Mr. Mamdani’s policy proposals aside. Mr. Trump crossed party lines to endorse Mr. Cuomo on Monday and threatened to withhold federal funds from the city if it elected “communist” Mamdani.

More than two million New Yorkers cast ballots in the election — a record last beat in the 1969 mayoral contest. Mr. Mamdani, who will be the city’s first Muslim mayor, earned more than one million votes to Mr. Cuomo’s 854,995 and Republican Curtis Sliwa’s 146,137 votes. To put this in perspective, only 1.1 million New Yorkers voted in the last mayoral contest in 2021.

When Decision Desk HQ called the race for Mr. Mamdani 30 minutes after the polls closed, chatter at the Cuomo election night watch party at the Ziegfeld Ballroom in Midtown Manhattan turned to a hum. Mr. Sliwa had already conceded. The wall-mounted NY1 news livestreams got cut. The next song to cue on the evening’s classic rock playlist was Bon Jovi’s “Livin’ on a Prayer.”

“I’m going to vomit,” a woman standing near the bar said. A few coarse shouts aimed at Mr. Sliwa punctured the room. The Sun spoke with more than a dozen watch party attendees, some of who said they expected Mr. Mamdani to win, others who teared up.

Earlier in the evening, attendees told the Sun they were anxious and obsessively looking at turnout data. When the New York City Board of Elections posted to X a gif with fireworks, announcing turnout had exceeded two million, attendees said they were cautiously optimistic that Mr. Cuomo could pull off an upset win.

“I thought we had a shot,” a single Jewish mother from the Upper East Side who works in finance, Suzanne Murphy, told the Sun. “I lived here in the 70s. I know what that’s like. I don’t want to see that happen to New York again.”

Mr. Cuomo’s recent social media hire, influencer Zach Sage Fox, told the Sun he “1,000 percent” could have helped Mr. Cuomo win if he’d had two more months to create viral content for the candidate. Instead, he had less than a month on the job.

“I had gone to a lot of the super PACs, starting early in the summer, having a lot of conversations with just older people who were running these things with very dinosaur thinking about social media,” Mr. Fox said. “I had one of the heads of a Super PAC tell me, to my face, social media is not that important. I mean, this is insane.”

Mr. Mamdani’s rise from one percent in the polls to mayor-elect in less than a year is due in part to his viral vertical videos and social media prowess. His affordability message appealed to a growing contingent of over-educated, downwardly mobile elites in areas like Greenpoint, Ridgewood, and Brownstone Brooklyn. He capitalized on the anti-Israel movement.  

“I really hope the big takeaway, outside of specific issues, is that social media is the battlefield now,” Mr. Fox said.

A Democratic strategist, Michael Hardaway, tells the Sun that Mr. Mamdani’s meteoric rise is about more than social media. “I think this election is a referendum on the status quo, and voters are saying what currently is happening in the city no longer works for us,” he says. “If Democrats are smart, they’re going to ask themselves, why the excitement? How do you bottle excitement?”

Mr. Cuomo walked on stage to give his concession speech at 10:40 p.m., flanked by his three daughters. “This campaign was the right fight to wage,” he said, referencing his choice to run against Mr. Mamdani in the general election, after losing to him by 13 points in the Democratic primary in June.

“This campaign was to contest the philosophies that are shaping the Democratic Party, the future of this city, and the future of this country. This coalition transcended normal partisan politics,” he said.

The crowd booed when Mr. Cuomo congratulated Mr. Mamdani. “That is not right. That is not us,” Mr. Cuomo said.

Mr. Mamdani took the opposite tone. Any predictions that he would moderate once in office were quickly tossed to the side on Tuesday night, when the mayor-elect started his speech by quoting five-time Socialist Party of America presidential candidate, Eugene Debs. He then spoke in Arabic and reiterated his campaign promises of free childcare, free buses, and a rent freeze in a call and refrain with the audience.

Mr. Mamdani didn’t mention Mr. Sliwa by name, but he gloated about toppling the Cuomo “political dynasty” and sending the former governor back into “private life.” He mocked Mr. Cuomo by repeating a line made famous by his father, Governor Mario Cuomo: “while you campaign in poetry, you govern in prose.” It was a knife to the man he beat — twice in six months — as if to say you will never measure up to your father.

“All the conventional wisdom would tell you that I am far from the perfect candidate. I am young, despite my best efforts to grow older. I am Muslim. I am a Democratic socialist. And most damning of all, I refuse to apologize for any of this,” Mr. Mamdani said.

As Cuomo supporters streamed out of the ballroom, sadness turned to anger for some. Several attendees told the Sun that Mr. Cuomo should have fought back against Mr. Mamdani harder and earlier. Many attendees blamed Mr. Sliwa, even though Messrs. Sliwa’s and Cuomo’s votes combined still would not have beaten Mr. Mamdani.

“Curtis, rot in f__king hell,” the founder of Empire State Properties and a WABC radio host, Suzanne Miller, told the Sun. Ms. Murphy said she’s more saddened and worried. “The fight has only just begun,” she said. “Donald Trump will make this personal, and he will toy with this guy like a cat with a half dead cockroach — and it’s not going to be good for New York. And that’s what I care about.”