The prospect of Mr. Mamdani’s campaigning as the Democratic standard-bearer in a city where Democrats outnumber Republicans by six to one has sent shivers down the spines of many New York business leaders, who recoil at his plans for expansive new government programs funded with tax increases on corporations and the wealthiest New Yorkers. Some have quickly begun to throw their support behind the incumbent mayor, despite the scandals that have tarnished his tenure.

Andrew Epstein, a spokesman for Mr. Mamdani, said the businessmen at the meeting were simply scared of “our plan to tax them a little bit more to fund an agenda to lower the cost of living and improve the quality of life for all New Yorkers.”

Mr. Loeb, an outspoken proponent of charter schools, has a long history of supporting Mr. Adams and other centrist Democratic candidates in local New York races. In 2021, he donated $1 million to a super PAC supporting Mr. Adams’s successful bid for mayor. He donated $350,000 to a super PAC supporting Mr. Cuomo in this year’s primary. On Wednesday, after Mr. Mamdani appeared triumphant in the race, Mr. Loeb posted to social media, “It’s officially hot commie summer.”

The previously unreported meeting between Mr. Loeb, Mr. Adams and others was one of several discussions among business leaders after Tuesday’s primary about how best to stymie Mr. Mamdani’s rise.

Scott Rechler, one of the city’s biggest landlords, on Wednesday told The New York Times that if the race pitted Mr. Mamdani against Mr. Adams, he would back the incumbent.