As the Democratic mayoral candidates heaped criticism on Mayor Bloomberg yesterday and took mild swipes at one another, they reserved some of their harshest language for figures unaccustomed to being targets of Democratic ire: leaders of organized labor.

Late in last night’s debate, the candidates were asked by a reporter for NY1, Rita Nissan, whether they felt the leaders of organized labor who had endorsed Mr. Bloomberg for re-election had offered their backing in error; whether they as candidates had hoped to receive the unions’ support, and whether they had tried hard enough to win it.

Earlier this month, Mr. Bloomberg received the endorsement of the city’s largest building workers’ union, Local 32BJ. The union, which represents more than 75,000 workers, backed the Democrat, Mark Green, against Mr. Bloomberg in 2001. Last month, the mayor snatched the endorsement of District Council 37, the city’s largest union of public employees, representing around 121,000 laborers.

The responses from those challengers to Ms. Nissan’s question last night were biting. The Democratic front-runner, Fernando Ferrer, said he was unconcerned with the opinions of labor leaders and was instead endeavoring to work for the rank-and-file members of the unions. Mr. Ferrer said he felt the unions’ membership disagreed with their leaders’ decisions, and he expressed confidence that they supported his candidacy.

The speaker of the City Council, Gifford Miller, trotted out the endorsements he has received from other unions, including New York’s United Auto Workers, before calling the leaders who had backed Mr. Bloomberg “misguided.”

And the congressman from a Brooklyn-Queens district, Anthony Weiner, who, like Mr. Miller, was a target of organized labor’s wrath earlier this year for his opposition to the West Side stadium, said the union leaders should “hang their heads in shame” for having backed Mr. Bloomberg and having turned their backs on the historically strong ties between Democrats and organized labor.

All four Democrats said they hoped the membership of the unions would back them, despite the leaders’ endorsements. A spokesman for DC37, Rudy Orozco, said last night that the union could not provide comment until this morning. A spokesman for 32BJ, Matt Nerzig, also declined to comment.

The large municipal unions that have battled the mayor on contracts this year, those representing the city’s teachers, police officers, and firefighters, have not yet made a mayoral endorsement.