President Trump is threatening to withhold federal funding from New York City if mayoral candidate Zohran Mamdani, a Democratic-Socialist, wins the mayoral election in November, a threat that critics suggest could be in violation of federal election laws.

Mr. Trump made the declaration on Truth Social on Monday, while also calling the 33-year-old Mr. Mamdani “the best thing to happen to our great Republican Party.” The president made a similar statement last week from the Oval Office, suggesting a Mayor Mamdani could be an effective foil for Republicans to run against in the midterms.

“He is going to have problems with Washington like no Mayor in the history of our once great City,” Mr. Trump posted about Mr. Mamdani. “Remember, he needs the money from me, as President, in order to fulfill all of his FAKE Communist promises. He won’t be getting any of it, so what’s the point of voting for him?”

“This ideology has failed, always, for thousands of years. It will fail again, and that’s guaranteed!” Mr. Trump posted.

Mr. Mamdani is running on an anti-Trump, tax-the-rich-and-white platform and promising to deliver free buses, free child care, a rent freeze, and city-run grocery stores to New Yorkers. The state assemblyman is leading in all major polls, as is expected of the Democratic nominee in a city where Democrats make up two-thirds of registered voters. Another 20 percent of city voters are unaffiliated, while only 11 percent are registered Republicans.

His top opposition is the state’s Democratic former governor, Andrew Cuomo, who is running as an independent in order to get on the general election ballot. Mayor Eric Adams dropped out of the race on Sunday. The Guardian Angels founder, Curtis Sliwa, is the Republican candidate.

It’s unclear if Mr. Adams was offered any private or public sector job in return for dropping out of the race or if he did so out of his own concern for the city and realization that he would lose badly in November. The mayor reportedly was in failed talks earlier this month with Trump administration officials for an ambassadorship.

The Cuomo campaign has been calling for the general election to be a two-person race since the summer, arguing that the former governor has the greatest shot at beating Mr. Mamdani, to whom he lost by 13 points in the closed Democratic primary.

Mr. Cuomo praised Mr. Adams on Sunday evening for dropping out of the race. His super PAC, Fix the City, has seen an uptick in interest from donors and supporters in the last 24 hours. A group of wealthy business leaders in the city met earlier this month to figure out how to consolidate the race, with the theory that most of Mr. Adams’s voters will switch their votes to Mr. Cuomo, though that remains to be seen.

Mr. Adams was lagging in the polls, with an average of only 10 percent support. Mr. Cuomo is trailing Mr. Mamdani by between 15 and 20 percent in polls. Even if all of Mr. Adams’s supporters voted for Mr. Cuomo it would not propel him to victory — if the polls are to be believed. The polls failed to capture Mr. Mamdani’s support in the June primary.

“We have known since Andrew Cuomo decided after the June primary to remain in the race as an independent that this was a two-person general election contest,” a spokeswoman for Fix the City, Liz Benjamin, tells The New York Sun. “Everything else at this point is just a sideshow.”

Mr. Sliwa slammed the Cuomo campaign last week, accusing Cuomo backers of offering him money to drop out of the race and clear the field for the former governor to take on Mr. Mamdani one-on-one. On Monday, Mr. Sliwa said on “Good Day New York” that at least seven individuals offered him up to $10 million total to drop out, even promising to support his animal welfare projects and provide a headquarters for the Guardian Angels. He declined to name names.

“You can’t bribe me, buy me, lease me. I’m not for sale,” Mr. Sliwa said.

If Mr. Mamdani wins and Mr. Trump tries to make good on his threat to withhold federal funding, it will likely end up in the courts. New York state won its lawsuit last week against the Department of Homeland Security for withholding disaster relief funding over the state’s sanctuary city policies.

Critics say the president could be in violation of the Constitution by threatening to withhold funds, claiming that political retaliation violates voters’ First Amendment rights to free political expression and association. 

The 10th Amendment also states that what is not in the federal government’s power belongs to the states, suggesting it cannot coerce or punish local communities for electing officials disliked by the federal government. A federal judge blocked the Trump administration in August from withholding federal funds from dozens of sanctuary cities, saying the Constitution grants the “power of the purse” to Congress, not the president.

Mr. Mamdani welcomed the skirmish on Monday.

“I think that Donald Trump is going through the stages of grief. He began with denial, where he said there was no way we could win this race, and he would use every tool at his disposal to ensure that that was the case,” Mr. Mamdani told reporters. “We saw that with phone calls that he had with Andrew Cuomo and briefings that he had with pollsters. Now he’s coming to terms with the fact that we’re going to win this race.”

Mr. Trump, a native New Yorker, has not endorsed a candidate in the mayoral race, though he reportedly spoke with Mr. Cuomo over the summer. Mr. Cuomo denies this and says he does not want the president’s endorsement.

Earlier this month,Mr. Trump called Mr. Sliwa “not exactly prime time” and criticized his proposal to house cats in Gracie Mansion. Mr. Sliwa is also running on an independent “Protect Animals” line.