NEW YORK (Reuters) -Self-described democratic socialist Zohran Mamdani's unexpected upset in New York City's Democratic mayoral primary on Tuesday exhilarated progressive activists, who had banded together to prevent the more moderate former New York Governor Andrew Cuomo from triumphing.

But the surprise outcome also generated excitement from a very different group of people: national Republicans.

Soon after it became clear that Mamdani, a 33-year-old state lawmaker, was likely to prevail, Republican Vice President JD Vance sent congratulations on social media to the "new leader of the Democratic Party." The Republican's congressional campaign arm called him an "antisemitic socialist radical" and promised to tie him to every vulnerable Democrat in next year's midterm elections.

And on Wednesday, Republican President Donald Trump - a native New Yorker - piled on, writing on social media, "It's finally happened, the Democrats have crossed the line. Zohran Mamdani, a 100% Communist Lunatic, has just won the Dem Primary, and is on his way to becoming Mayor."

The reactions underscore both the risks and the rewards for the Democratic Party - still trying to find its footing five months into Trump's term - in having an unabashed left-wing nominee running in the country's biggest city this fall.

Mamdani's campaign, which drew plaudits for its cheery tone and clever viral videos, could help energize young voters, a demographic that Democrats are desperate to reach in 2026 and beyond. His rise from a virtual unknown was fueled by a relentless focus on affordability, an issue Democrats struggled to address during last year's presidential race.

"Cost of living is the issue of our time," Neera Tanden, the chief executive of Democratic think tank Center for American Progress wrote on X in response to Mamdani's win. "It's the through line animating all politics. Smart political leaders respond to it."

His history-making candidacy - Mamdani, born in Uganda to Indian parents, would be the city's first Muslim and Indian American mayor - could also drive engagement among Asian and especially Muslim voters, some of whom soured on the party after the Biden administration's support for Israel's war in Gaza.

"These elections aren't about left, right or center, they're about whether you're a change to the status quo. People don't want more of the same, they want someone who plays a different game," said Democratic strategist Jesse Ferguson.

But Mamdani's criticism of Israel and his democratic socialism are also likely to show up frequently in Republican attack ads. Much of the Democratic establishment had lined up behind Cuomo, including former President Bill Clinton, partly out of unease over Mamdani's platform. Mamdani has repeatedly said he is not antisemitic.

"I think he's an easy target for Republicans who want to use scare tactics to talk about the Muslim mayor from New York City who's uber-left," said Patrick Egan, a political science professor at New York University. But Egan noted, Mamdani has also proven to be an adept politician.

"When people get exposed to this guy, they tend to like him," he said.

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Basil Smikle, a political analyst and professor at Columbia University's School of Professional Studies, said heavy-handed attacks on Mamdani could backfire by energizing "a lot of the Democratic voters to want to push more against Trump."

"I don't think it hurts Democrats in the long run," he said. "I actually think it helps them."

For his part, Mamdani seemed ready to embrace his role as a party leader, telling supporters in his victory speech that he would govern the city "as a model for the Democratic Party - a party where we fight for working people with no apology." He vowed to use his mayoral power to "reject Donald Trump's fascism."

Democratic voters say they want a new generation of leaders and a party that concentrates on economic issues, according to a Reuters/Ipsos poll earlier this month.

"The Democratic Party is trying to figure itself out," said Christina Greer, a political science professor at Fordham University in New York.

While Mamdani enters the general election as the favorite in a city dominated by Democrats, the race is more unsettled than usual.

Mayor Eric Adams, a Democrat, is running as an independent after his popularity plummeted following his indictment on corruption charges and the subsequent decision by Trump's Justice Department to drop the case. Cuomo also retains the ability to run as an independent, though he has not yet decided whether to do so.

The Republican candidate is Curtis Sliwa, the founder of the Guardian Angels. Jim Walden, a former federal prosecutor, is running as an independent, as well.

The primary had become a two-man race by Election Day between Mamdani and Cuomo, echoing other Democratic nominating contests in which the party's establishment and liberal wings have wrestled for power. But it was also a generational clash between Mamdani and Cuomo, the 67-year-old scion of a New York political family.

That said, Cuomo carried plenty of personal baggage, four years after he resigned the governorship amid allegations of sexual harassment, which he has denied.

"Some people were voting for Mamdani to express their displeasure for Cuomo," Greer said.

Mamdani's unlikely ascension bore some of the same hallmarks of similar rises for two other democratic socialists, U.S. Senator Bernie Sanders and U.S. Representative Alexandria Ocasio-Cortez, both of whom endorsed his campaign.

Sanders, an independent, emerged as a leading Democratic presidential candidate in 2016 and 2020, while Ocasio-Cortez pulled off an upset in 2018 by defeating a longtime incumbent Democrat.

(Reporting by Joseph Ax and Maria Tsvetkova; additional reporting by James Oliphant and Helen Coster; editing by Paul Thomasch and Diane Craft)

By Joseph Ax and Maria Tsvetkova