Two powerful New York City labor unions that had supported former Gov. Andrew M. Cuomo in this year’s race for mayor have decided to abandon him and endorse Zohran Mamdani, the 33-year-old state assemblyman who has a commanding lead in the Democratic primary.

The two endorsements, along with one from a third union that did not back a candidate in the primary, seemed to be a clear sign that traditional Democratic power brokers are beginning to consolidate behind Mr. Mamdani.

Leaders of the three unions, the Hotel and Gaming Trades Council, Local 32BJ and the New York State Nurses Association, said they were supporting Mr. Mamdani, a democratic socialist, because he had made affordability his campaign’s centerpiece. They promised to invest in boots-on-the-ground campaigns to help him beat Mayor Eric Adams in November.

The switch to Mr. Mamdani may also be a nod to political reality. While this general election is poised to be particularly heated, the Democratic candidate for mayor is generally considered the heavy favorite to win City Hall in a city where Democrats outnumber Republicans by six to one.

“We are confident that whenever we’re in a fight, Zohran will be on our side standing up for hospitality workers,” said Rich Maroko, the Hotel and Gaming Trades Council president. “That’s why we are genuinely excited to endorse Zohran and ready to help him win in November.”

Theodore Schleifer contributed reporting.