Not long after Assemblyman Zohran Mamdani’s decisive victory in the Democratic primary for mayor of New York City, a group of wealthy real estate titans and financiers pledged to raise tens of millions of dollars to defeat Mr. Mamdani, a democratic socialist.

Their push has only become more urgent as the Nov. 4 election approaches, with one recent email warning business leaders that if they “fail to mobilize, the financial capital of the world risks being handed over to a socialist this November.”

So far, their pleas have yet to be answered, largely because of Mr. Mamdani’s popularity and the scattered nature of his opposition: Three rivals — former Gov. Andrew M. Cuomo, Mayor Eric Adams and Curtis Sliwa — remain in the race, splitting up supporters and donors opposed to Mr. Mamdani.

Even so, the effort to damage his campaign has begun.

A coalition of groups have shelled out hundreds of thousands of dollars in coordinated advertisements and canvassing efforts aimed at blunting Mr. Mamdani’s momentum. Ads and campaign palm cards paint him as an idealistic political novice whose policies would harm the working class New Yorkers he proclaims to help.

One such digital advertisement warns against putting “a social media personality in charge of a $110 billion budget and eight million people,” as photos and videos taken from Mr. Mamdani’s short-lived career as a rapper flash across the screen.

“Mamdani’s viral posts might be catchy,” the video says. “But his inexperience will be chaos.”

Dora Pekec, a spokeswoman for Mr. Mamdani, downplayed the effort.

“Billionaires may think that everything is for sale but you can’t buy enthusiasm,” she said. “The reason tens of thousands of New Yorkers will have knocked on millions of doors by Nov. 4 is because they’re hungry for a different kind of politics and a city government relentlessly focused on making life here affordable.”

The anti-Mamdani coalition does face a daunting challenge. The assemblyman is leading by roughly 20 points in recent polls of the mayoral race, with Mr. Cuomo in second, followed by Mr. Sliwa, a Republican, and Mr. Adams. Mr. Cuomo and Mr. Adams, both Democrats, are running on third-party lines.

Fund-raising by anti-Mamdani super PACs has also been sluggish. The largest group, Fix the City, raised $25 million to oppose Mr. Mamdani and boost Mr. Cuomo during the Democratic primary. Since then, it has raised just $2.8 million, according to the New York City Campaign Finance Board.

A sizable chunk of that, $750,000, came from Ronald Lauder, the on-again, off-again ally of President Trump who helped spark his interest in Greenland. A cosmetics heir, Mr. Lauder has donated millions of dollars to elect Republicans and defeat progressive measures statewide. In this race, however, he appears to have thrown his support behind Mr. Cuomo.

Fix the City also received $50,000 this month from Deborah Simon, the daughter of an Indiana shopping mall mogul who has historically funded left-leaning causes, like Planned Parenthood, and the Democratic National Committee.

Other large donors — like Bill Ackman, the hedge fund billionaire and Trump donor, and former Mayor Michael R. Bloomberg — gave generously in the primary but have not provided aid to the Cuomo-affiliated super PACs in the fall.

Mr. Bloomberg has even met with Mr. Mamdani, though the billionaire made clear that the meeting was not meant to be interpreted as an endorsement.

Fix the City recently combined forces with two newer, smaller super PACs, Protect the Protectors and New Yorkers for a Better Future Mayor 25, creating an anti-Mamdani coalition that can coordinate spending to chip away at the Democratic nominee’s lead.

Fix the City is planning to focus on the Bronx, where Mr. Cuomo outperformed Mr. Mamdani in the primary by 20,000 votes. The group has paid canvassers to knock on doors and hand out palm cards that highlight Mr. Cuomo’s accomplishments on gay marriage and infrastructure; the cards also hit at Mr. Mamdani’s relative inexperience, declaring: “New York City Mayor is not an entry-level job.”

The push is part of an effort to incorporate some of the lessons of the primary, when the PAC spent big on expensive television ads that failed to lift Mr. Cuomo to victory, said Liz Benjamin, a spokeswoman for Fix the City.

Some well-heeled donors who oppose Mr. Mamdani have been reluctant to engage in the general election until it becomes clearer that he can be beaten. The best chance of that happening is if the crowded field thins so that one major candidate can face Mr. Mamdani head-to-head.

But despite a drumbeat of rumors of backroom deals and plum job offers, neither Mr. Adams nor Mr. Sliwa have publicly signaled any interest in stepping aside.

Even so, those leading the effort against the Democratic nominee say they still see an opening to clear the field. They argue that the more voters learn of his thin policy record and past comments about critical city services like law enforcement, the less inclined they will be to support him.

“It has everything to do with his positions in the Assembly, votes he’s taken, who he really is, what he’s really said,” said Hank Sheinkopf, a longtime Democratic consultant who leads Protect the Protectors. “He and his handlers have done a brilliant job of trying to make him into somebody he’s not.”

Save NYC, a group led by a former Republican lieutenant governor, Betsy McCaughey, has raised roughly $1.3 million and started doing anti-Mamdani canvassing directly to city homeowners. The group argues that his support for Local Law 97, a measure that currently requires climate-friendly building upgrades, will create a financial burden that could possibly price them out of their homes altogether.

Ms. McCaughey said in an interview that Mr. Mamdani’s support for the law was shortsighted and underscored his lack of experience. Her group, which launched its canvassing efforts at several condominiums and co-ops in Queens on Thursday, hopes to reach 1.4 million homeowners citywide.

She said that “1.4 million voters is three times the number of voters who turned out for Zohran Mamdani in the primary. Even if we get half of them to vote in their own economic self-interest, he is defeated.”

New Yorkers for a Better Future Mayor 25, recently spent $250,000 on digital ads that took the unusual approach of acknowledging Mr. Mamdani’s appeal. The ads, which drew attention after it became known that the names of voters were changed and stock photos were used, are aimed at people who supported Mr. Mamdani in the primary but may not be aligned with him on issues like education and prostitution.

“He has a nice smile,” a Black man identified as Desmond, 41, from Harlem begins, before raising issues with Mr. Mamdani’s rent freeze proposal.

“He’s a good man with bad ideas,” begins another, featuring an Asian woman identified as Ling, 51, from Chinatown.

Jeffrey Leb, the group’s treasurer, says the upbeat approach is poll-tested but had not been deployed in the 2025 mayoral race.

“The doom-and-gloom negatives don’t work, people just gloss over it,” he said. “But if we open up with like, ‘hey, you know, he’s a nice guy, but he just has no freaking experience.”

Benjamin Oreskes and Nicholas Fandos contributed reporting.