The ballroom is packed with men wearing blue jeans and T-shirts, waving hard hats, American flags, and signs saying “Unions for Mike Bloomberg.”

“You want a mayor who understands you, who understands what you do,” a union leader, standing on a stage at the front of the room, says. The room breaks into cheers of “Four more years,” punctuated by hoots and whistles. A heavy-set man appears on a video screen at the front of the room and says, “Mayor Bloomberg is for working men. That’s a fact.” The men in the room go wild.

When the billionaire mayor himself arrives at the rally, wearing a crisp white shirt and blue tie but no jacket, camera flashes light up the room, a powerful bass makes the floor shake, and screams, hollers, and whistles follow him as he approaches the stage.

The mayor – who has engaged in heated and sometimes unsuccessful negotiations with the city’s labor unions – was greeted like Hilary Duff entering a room of preteen fans yesterday afternoon, as he received the endorsement of the District Council of Carpenters, the New York State Laborers Union, and District Council 9, Painters. Yesterday’s endorsements came on the heels of a Wednesday endorsement by District Council 37, the largest municipal union.

Although many important union endorsements, including those of the health-care union, 1199SEIU, the building service workers union, Local 32BJ, and the United Federation of Teachers are still up for grabs, Mr. Bloomberg is proving that although he is a Republican and has been on less than friendly terms with some unions, he is more than capable of winning labor endorsements.

“I would say that getting DC-37 is an exceptionally good start,” a political consultant, Norman Adler, who was the director of political action and legislation for DC-37 for 11 years, said. “Symbolically they’re important. The guy is a billionaire, who in the past was accused of being out of touch.”

Mr. Adler said union endorsements and labor rallies help Mr. Bloomberg show that he is not distant from workers.

“It’s the Bloomberg campaign’s effort to say, ‘See, this is not an out-of-touch billionaire. It’s a regular guy,'” he said.

Although Mr. Adler said support from DC-37 and some of the smaller unions would help Mr. Bloomberg – especially if the election is close – he said the support of one union does not guarantee support from other unions.

“I don’t think there’s any labor solidarity motive,” he said. “Each union’s going to do what’s good for itself.”

The chairwoman of New York’s Municipal Labor Council, Randi Weingarten, said “pragmatism” explains the endorsements that Mr. Bloomberg has received to date.

“The unions are being pragmatic,” she said. “They believe he is going to win and they want to be on board. In some cases there’s a real chemistry there. In others it’s just pragmatism.”

Ms. Weingarten, who is also president of the teachers union, is locked in a bitter contract dispute with the mayor.

Although she has met with many of the Democratic mayoral candidates, her union has not endorsed anyone, and she has said her main goal in the coming months is negotiating a new contract for her members. The teachers contract has been expired for more than two years.

A senior research associate at the National Institute for Labor Relations Research, Stan Greer, also said the unions that are coming out for Mr. Bloomberg at this point – long before the Democratic primary and four months before the general election – are acting in self-interest.

“The calculation is it’s very likely that he’s going to be the mayor for four more years,” Mr. Greer said. “You’ll get a better deal going along with him now.”

Leaders of the painters, carpenters, and laborers who turned out to yesterday’s rally said they were not offering their support to gain political favors. Rather, they said, they were supporting a mayor who has spent four years boosting economic development and creating jobs for union workers.

“We don’t endorse candidates just for the sake of endorsing,” the business manager of the Mason Tenders District Council, a branch of the New York State Laborers Union, Robert Bonanza, said. He said Mr. Bloomberg has helped labor during his time at City Hall and now it’s time for labor to “be there with him.”

The business manager of District Council 9, Painters, Sandy Vagelatos, said four years ago he never would have expected to endorse Mr. Bloomberg, but he said the mayor has helped unions. Mr. Vagelatos said he wanted to send a message with his union’s endorsement to politicians who “use us today and abuse us tomorrow.”