Despite negotiating generous contracts with the city under Mayor Michael R. Bloomberg, the largest firefighters union on Wednesday endorsed Mr. Bloomberg's Democratic opponent, Comptroller William C. Thompson Jr., for mayor.
Stephen Cassidy, the president of the Uniformed Firefighters' Association, which represents 8,900 firefighters, praised Mr. Thompson as "smart, tough and fair'' and directed harsh criticism at Fire Commissioner Nicholas Scoppetta, who was appointed by Mr. Bloomberg.
"The Fire Commissioner of the city of New York has always set accountability for firefighters but not for himself," Mr. Cassidy said. "Eight years under Mike Bloomberg we've heard Nick Scoppetta say, 'It's not my fault.'"
Mr. Cassidy said he held Mr. Scoppetta accountable for the deaths of two firefighters two years ago at a fire at the Deutsche Bank Building in Lower Manhattan. A report by the Manhattan district attorney's office said several city agencies had failed to properly monitor the building, where a basement standpipe meant to deliver water to firefighters had been disabled, apparently without the Fire Department's knowledge.
Mr. Cassidy said Mr. Bloomberg had sought the support of his union, which had remained neutral in the race for mayor four years ago. But Mr. Bloomberg's re-election campaign denied seeking or expecting the union's endorsement.
For Mr. Thompson, the union's endorsement is a welcome boost from labor as he faces an uphill climb against a much better-financed opponent.
Mr. Thompson already won the backing last month of New York City's largest municipal union, District Council 37, which represents 120,000 city government voters.
In an e-mail message, a spokeswoman for the Bloomberg campaign, Jill Hazelbaker, said: "New Yorkers care about results, not rhetoric. Under Mayor Bloomberg, the city has invested in promoting the use of smoke detectors, educated hundreds of thousands of New Yorkers on fire prevention, responded to fires faster than ever before, and there have been fewer fire-related deaths this year."
Fire Department officials also defended Mr. Scoppetta's record and said that the department's performance had improved during his tenure.
"There's no question that today the fire department is better trained, better prepared for any emergency," said Deputy Commissioner Francis X. Gribbon. "The public has never been better served by the department and its management and its effectiveness."
As rows of firefighters stood behind him, Mr. Cassidy criticized the mayor for raising the idea earlier this summer that 16 firehouses might have to be closed, a notion averted by a budget deal with the City Council. He also criticized Mr. Bloomberg for his push to persuade the City Council to change the term limits law so he could run for a third term.
Mr. Thompson echoed Mr. Cassidy's point. "The voters of New York City are not going to forget," Mr. Thompson said. "Mike Bloomberg broke his word to the people of New York City."