Former New York City Mayor Rudy Giuliani told Newsmax on Wednesday that his “disappointment is immense” with the election of Democrat socialist Zohran Mamdani as mayor.

“It’s almost impossible for me to express it,” Giuliani said in an interview on Newsmax’s “American Agenda.” “My disappointment is immense. I feel like all the work that I did has been ruined and will be ruined, and [Mike] Bloomberg’s as well.”

He added that Mamdani got off to a “terrible” start by attacking President Donald Trump during his acceptance speech, when the mayor-elect said: “Donald Trump, since I know you’re watching, I have four words for you: Turn the volume up.”

“The stupidest, dumbest, most childish thing you can do as a mayor is to attack the president of the United States,” Giuliani said.

He added that when he became mayor in 1994, after the election of Democrat President Bill Clinton, he put political differences aside for the good of New Yorkers.

“I didn’t vote for Bill Clinton,” Giuliani said. “I campaigned very, very, very hard for President [George H.W.] Bush. Not only was he my president, but he was a good friend.”

Still, said Giuliani, “I did everything to work with President Clinton.”

“I have a letter from President Clinton that I was the best mayor in America because I worked with him better than Democrat mayors did for the good of my city,” said the former mayor.

He said cooperation, not confrontation, defined his approach.

“I also sued him once because he was taking money from us he shouldn’t, and he understood it,” Giuliani said.

Clinton, however, “was a professional,” said Giuliani. “But I’ve never heard of a mayor attacking a president.”

The former mayor said such attacks could have serious consequences for New York if the federal government were to take a closer look at how the city uses taxpayer money.

“If the president of the United States just asked his administration to enforce the law with regard to federal grants, you could probably cut off half to New York just on corruption alone,” said Giuliani.

The one-time mayor also referred to New York as the “most corrupt city in the country,” pointing to its spending levels.

“The budget of New York City, with 8 million people, is equal to the budget of the state of Florida, with 22 million people,” he said. “Now, how can that be? Because New York has an awful, awful lot — I mean billions — in kickback money.”

Giuliani said he witnessed that corruption firsthand when he was U.S. attorney before becoming mayor.

“I know, because I put a lot of them in jail when things were cheaper and we were talking millions then, but it’s been corrupt for 160 years with a few interruptions,” he said.

He also noted how rare Republican leadership has been in the city’s modern history.

“I was only the fourth Republican mayor to get elected in the 20th century, and I was only one of two that remained a Republican: [Fiorello] La Guardia, you heard of him, and me,” Giuliani said.

He also expanded his criticism beyond City Hall, warning about what he described as extremist influences surrounding the new mayor.

“We concentrate on his being a communist,” he said. “He’s also a very strong supporter of Islamic extremism.”

Giuliani also said many young people who voted for the city’s new leadership don’t know the full story of New York’s past, including the 9/11 attacks.