ayor Michael R. Bloomberg won the endorsement yesterday of the Building and Construction Trades Council of Greater New York, a consortium of construction unions. The group says it represents 100,000 members who live in the five boroughs, many of whom will work the phones and the streets through the fall for Mr. Bloomberg's re-election.

The mayor's aides said the endorsement was evidence of the little-discussed political benefits of his contested plan for a West Side stadium: Though it remains unpopular with many voters and may be costing Mr. Bloomberg political support from them, according to polls, it happens to be exceptionally popular with the construction unions.

Indeed, Edward J. Malloy, the head of the construction unions council, said Mr. Bloomberg's drive for the stadium - which would serve the New York Jets, potentially the 2012 Summer Olympics and various conventions - was one of the main factors in its endorsement of him.

"The stadium is important. The Olympics is important," Mr. Malloy said. "But there's a lot of other projects that are in the pipeline that will be built."

The council, whose membership Mr. Malloy described as "Reagan Democrats," has a history of endorsing Republicans, backing Rudolph W. Giuliani for mayor and George E. Pataki for governor in recent years. But the union backed the Democratic mayoral candidate in the 2001 general election, Mark Green, over Mr. Bloomberg.

"We're practical," Mr. Malloy explained. "We thought that Mike Bloomberg didn't have a chance; so we were proven wrong."

The endorsement provided Mr. Bloomberg with much-needed labor support, as he faces anger from several municipal unions that are working without contracts as negotiations grind on. Some groups, like the United Federation of Teachers, have even run commercials slamming him.

Asked if he felt at all as if he was being disloyal to his fellow labor leaders by backing the mayor at a time when other unions are battling him, Mr. Malloy said he did not because "I think eventually everybody will be treated fairly."

Mr. Malloy's group gave its backing to Mr. Bloomberg at a morning ceremony at the construction site of a new dormitory for the Brooklyn Law School.

It was the first time this election season that Mr. Bloomberg had ventured out of City Hall during a normal workday for an overt campaign event. And the ceremony provided the sort of visual that any political image-maker would want to use in a commercial: a slightly tanned Mr. Bloomberg, standing amid a group of burly hard-hat workers of all races, with his shirtsleeves rolled up.

But if the event left any doubts that Mr. Bloomberg was in campaign mode, his appearance on "Late Night With Conan O'Brien" - which was taped to be shown early this morning - should have dispelled them.

The mayor joked about his experiences riding the subway to City Hall and some of the bizarre calls that have been made to the city's 311 information hot line, like the one from a person who wanted Oprah Winfrey's telephone number.

Mr. Bloomberg also talked about his own habit of making calls to 311 to report problems that he encounters while out and about in the city. He said he called a few weeks ago after seeing some trash in the street.

"The young lady said, 'What's your name?' And I said, 'Bloomberg,' and she said, 'How do you spell that?' " he said, drawing laughter.