A representative for Mr. Trump, who has not made an endorsement in the race, confirmed the meeting.
Mr. Mateo has repeatedly voiced his support for the former president, who is under investigation by the Manhattan district attorney.
Throughout his campaign, Mr. Mateo has criticized Mr. Sliwa, the founder of the Guardian Angels who only became a Republican last year, for not supporting or voting for Mr. Trump, who remains popular with Republicans.
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The two have also sparred over the lie that Mr. Trump won the 2020 election — Mr. Sliwa says he did not — which has become a litmus test for conservative candidates across the country.
Mr. Flynn, a former general who became one of the most ardent voices in Mr. Trump’s push to overturn the election and recently suggested at conference organized by adherents of the QAnon conspiracy theory that he would support a military coup, cited Mr. Mateo’s embrace of the former president as the reason for his endorsement.
“He understands, supports and embraces President Trump’s America First agenda,” Mr. Flynn said.
McGuire’s wife cuts an ad
Mr. Yang’s wife, Evelyn, rode the Cyclone roller coaster in Coney Island, Brooklyn, with him in his first advertisement.
Mr. Stringer’s wife, Elyse Buxbaum, appears with him in an ad showing the couple getting their two sons ready for school.
Now, as the former Wall Street executive Raymond J. McGuire continues to struggle in the polls, his wife, Crystal McCrary McGuire, a lawyer and filmmaker, is appearing solo in an ad set to launch on Tuesday.