The emergence of Zohran Mamdani, a democratic socialist, is likely to divide national Democrats, who are already torn about what the party should stand for.
The national Democratic establishment on Tuesday night struggled to absorb the startling ascent of a democratic socialist in New York City who embraced a progressive economic agenda and diverged from the party's dominant position on the Middle East.
As elections go, Tuesday's party primary for mayor was a thunderbolt: New York voters turned away from a well-funded familiar face and famous name, former Gov. Andrew M. Cuomo, and in doing so made a generational and ideological break with the party's mainstream. They turned to a 33-year-old, three-term state assemblyman, Zohran Mamdani, who ran on an optimistic message about affordability and the rising cost of living that has eluded many national Democrats.
What became vividly clear on Tuesday, as votes were counted across the racially and economically diverse neighborhoods of New York, was that Mr. Mamdani had generated excitement among some -- though not all -- of the traditional pillars of winning Democratic voter coalitions.
Democratic leaders badly want to win over young voters and minority groups in the coming 2026 and 2028 elections -- two groups they have struggled to mobilize since the Obama era -- but they also need moderate Democrats and independents who often recoil from far-left positions.
"It really represents the excitement that I saw on the streets all throughout the City of New York," said Letitia James, the New York attorney general. "I haven't seen this since Barack Obama ran for president of these United States."
That Mr. Mamdani had such success while running on a far-left agenda, including positions that once were politically risky in New York -- like describing Israel's actions in Gaza as genocide and calling for new taxes on business -- may challenge the boundaries of party orthodoxy and unnerve national Democratic leaders.
His views on Israel are likely to force some national Democrats into an uncomfortable position, said David Axelrod, a native New Yorker and Mr. Obama's chief strategist. But he said that Mr. Mamdani's relentless focus on economic affordability resonated widely and could be a playbook for the party's success as well.
"There is no doubt that Trump and Republicans will try and seize on him as a kind of exemplar of what the Democratic Party stands for," Mr. Axelrod said. "The thing is, he seems both principled and agile and deft enough to confront those sort of conventional plays."
The primary results raised questions about how Democrats on the national level would react. Would they embrace Mr. Mamdani as a next-generation leader of the party who can articulate a resonant economic message in a way that former Vice President Kamala Harris failed to do in November? Or would they distance themselves from his democratic socialist ideas and move more directly toward the center to court independent voters?
The enthusiasm that Mr. Mamdani generated among a swath of New Yorkers looking for fresh leadership called to mind the insurgent campaigns of Senator Bernie Sanders in the 2016 presidential race and Representative Alexandria Ocasio-Cortez in her upset victory in a 2018 House primary.
All three are democratic socialists, a once fringe movement popularized by Mr. Sanders that calls for reining in the excesses of capitalism and curbing the power of the wealthy.
Mr. Sanders and Ms. Ocasio-Cortez, both of whom endorsed Mr. Mamdani, remain popular progressive figures but have had only limited impact on changing the Democratic Party's agenda and messaging.
A key question is how the Democratic donor class and business community, which was already unsettled by Mr. Mamdani's rise, will react to his apparent victory. Business leaders may flock to his rivals in the general election in November, or try to use super PACs to stop him.
In the meantime, other Democratic elected officials are likely to be questioned on whether they agree with his positions.
"It's a national election, not just a New York City election. People are going to be watching," said James Carville, a longtime Democratic strategist. "Everybody will have to weigh in one way or another. Everybody is going to be asked, do they support him."
Even before Mr. Mamdani addressed his supporters early Wednesday morning, Republicans were gearing up to caricature him.
The National Republican Congressional Committee gleefully declared Mr. Mamdani the "new face of the Democrat Party." Senator Rick Scott of Florida predicted on social media that more New Yorkers would be fleeing to his state. And Representative Elise Stefanik of New York sent a fund-raising appeal on Tuesday saying her "stomach was in knots," calling Mr. Mamdani a "Hamas Terrorist sympathizer." (Mr. Mamdani has defended pro-Palestinian slogans like "globalize the intifada." He has said that he supports an Israel with equal rights for all its citizens but has not said if it has a right to exist as a Jewish state. He has emphatically denied accusations that he is antisemitic.)
As recently as last month, few people expected Mr. Mamdani to beat Mr. Cuomo, 67, who benefited from near universal name recognition, a deep war chest, and the endorsement of party heavyweights like former President Bill Clinton.
But the embrace of the party establishment may have done Mr. Cuomo no favors in a race that appeared to be marked by a deep hunger for change.
"Voters are not happy with the national party establishment and want to focus on building a movement," said Basil Smikle, a professor at Columbia's School of Professional Studies. "I think that's key here. Mamdani created a movement around his candidacy."
Mr. Mamdani ran a relentless and cheerful campaign focused on affordability in a city that has grown too expensive for an expanding circle of residents, with zippy videos and catchy tag lines like "freeze the rent" and "free buses" that told voters he cared first about their wallets.
That kitchen-table focus actually mirrored economic messaging that some in the center of the party have also urged Democratic candidates to embrace. Many Democrats were frustrated that did not occur with former President Joseph R. Biden Jr. and Ms. Harris during the 2024 election.
David Shor, a Democratic researcher and strategist who worked with the leading pro-Harris super PAC, Future Forward, wrote on X that Mr. Mamdani was "a great example of how far you can go if you genuinely center your campaign in an engaging way around the issue that voters overwhelmingly say in surveys they care the most about."
Benjamin Oreskes, Reid J. Epstein and Shane Goldmacher contributed reporting.