Two influential New York City labor unions that backed Mayor Eric Adams in 2021 switched their support on Monday to former Gov. Andrew M. Cuomo, reflecting his growing dominance as the race for mayor accelerates.

The coveted endorsements came from the Hotel and Gaming Trades Council and Local 32BJ of the Service Employees International Union, which represents building workers. Together, the unions have more than 125,000 members, and they typically spend millions of dollars supporting their chosen candidates.

Both unions have a contentious history with Mr. Cuomo. They worked with him to pass a statewide increase to the minimum wage and other policies as governor, but later called for his resignation in 2021 amid mounting sexual harassment accusations. (Mr. Cuomo, who resigned, denies any wrongdoing.)

Now, they have concluded that Mr. Adams is fading politically and Mr. Cuomo is on an increasingly direct path to City Hall. And like much of the city’s Democratic establishment, the unions appear more interested in making amends than antagonizing a famously sharp-elbowed leader who could soon have influence over city contracts and other union priorities.

In a joint endorsement on Monday, the union leaders praised Mr. Cuomo as a steady supporter of working New Yorkers and the kind of leader who could stand up to President Trump as he threatens to withhold federal funding from cities like New York and undermine labor rights.

“As Andrew Cuomo said when he recently addressed our members, when we need him in a fight, he will be in the foxhole with us until the end,” said Rich Maroko, the head of the hotel and casino union.

Mr. Maroko did not address his change of heart since four years ago, when he said that Mr. Cuomo was not fit to be governor. He did, however, acknowledge that Mr. Cuomo could be an important ally as the union negotiates a new citywide hotel contract.

The endorsement was as much a boon to Mr. Cuomo, 67, as it was a blow to his rivals. With just over two months to the primary, they urgently need fresh momentum to chip away at his steady lead in public opinion polling and had hoped the unions might help.

As Mr. Cuomo rolled out his newest supporters, his rivals were lampooning a rare blunder by his campaign, which posted a housing plan that included garbled passages and appeared to draw, at least in part, on material collected by ChatGPT, news that was first reported by Hell Gate, a local news site.

“I did the hard work to pass city laws that will create 120,000 new housing units,” Adrienne Adams, the City Council speaker and another candidate in the Democratic primary for mayor, wrote on X. “Andrew Cuomo asked ChatGPT what his housing policy should be. Guess someone does need on-the-job training.”

The campaign made more sloppy mistakes on Monday when it announced the union endorsements. Its news release misspelled the names of Mr. Maroko and Manny Pastreich, the president of 32BJ, in bold-faced type summarizing the news.

Still, it was hard for other candidates to entirely sidestep the sting of losing out on the support of two unions known in New York City for their political influence.

In 2021, the hotel and building workers’ unions were at the heart of a working-class coalition that helped propel Mr. Adams to victory, and they have worked closely with his administration.

This time, though, both unions concluded that Mr. Adams had no path to re-election. After courting Mr. Trump’s help to shake federal corruption charges, Mr. Adams decided to skip the Democratic primary and run as a political independent this fall.

The unions considered other Democrats, including Ms. Adams and Assemblyman Zohran Mamdani, a democratic socialist running a distant second behind Mr. Cuomo. But the candidates evidently failed to convince the labor leaders they could win.

Other unions have reached different decisions. The union representing public defenders, museum staff members and academics endorsed Mr. Mamdani, State Senator Jessica Ramos and Brad Lander, the city comptroller, in a joint endorsement. Ms. Ramos, the chairwoman of the State Senate’s Labor Committee, also has the support of some teamster groups.

Other large unions remain undecided and could still shake up the race, including District Council 37, the city’s largest public employees union, and Local 1199 of the S.E.I.U, which represents health care workers. The longtime leader of the latter group, George Gresham, is said to personally support endorsing Mr. Cuomo but is facing a broader revolt within his union.

So far, Mr. Cuomo has consolidated the largest bloc of union support. He won earlier endorsements from unions representing New York City’s carpenters, electrical workers, painters and operating engineers.

The groups typically point to Mr. Cuomo’s long track record and moderate, pro-labor and pro-immigrant stances as governor to explain their support.

Mr. Cuomo thanked the unions that backed him on Monday, saying they had been “failed by their government for too long.”

“Without a strong middle and working class, no city can survive — let alone thrive — and right now, we are dangerously close to losing them,” he said.