In a letter to its members Friday, the International Association of Fire Fighters, excoriated Giuliani for his November 2001 decision to cut back the number of firefighters searching the rubble of Ground Zero for the remains of some 300 fallen comrades.
The 280,000-member union accused him of carelessly expediting the cleanup process with a "scoop-and-dump" operation after the recovery of millions of dollars in gold, silver and other assets from the Bank of Nova Scotia that had been buried.
Giuliani's campaign insisted that he respects and supports first responders.
The former mayor and the union have feuded for years over his policies in the aftermath of the attacks, but the firefighters' latest criticism comes as several polls show Giuliani ahead by wide margins in the GOP nomination race.
Seeking to blunt the impact of the accusations, his campaign announced the support of nearly 100 South Carolina firefighters and countered with its own letter from Lee Ielpi, a retired New York firefighter.
"There is no one who respects firefighters and first responders more than Rudy Giuliani," Ielpi wrote. "Firefighters have no greater friend and supporter."
The union's latest broadside initially was included in a scathing letter dated Feb. 28. Union officials say that letter was drafted as leaders were weighing whether to invite Giuliani to a presidential candidate forum but never was distributed to members because the union ultimately invited Giuliani. Giuliani, however, declined the invitation to next week's forum, citing scheduling conflicts.
"We decided to fall on the side of taking the high road and extend an invitation to him," said Harold Schaitberger, the union's general president. "That letter was never intended to be released."
Nevertheless, the letter showed up on Web sites this week. After it surfaced, the union decided to send a revised letter with the same criticisms to its members on Friday and posted it on the union's Web site.
"Mayor Giuliani's actions meant that firefighters and citizens who perished would either remain buried at Ground Zero forever, with no closure for families, or be removed like so much garbage and deposited at the Fresh Kills landfill," the letter said, adding: "Hundreds remained entombed in Ground Zero when Giuliani gave up on them."
"What Giuliani showed is a disgraceful lack of respect for the fallen and those brothers still searching for them," it added.
The union said the purpose of the letter was "to make all our members aware of the egregious acts Mayor Giuliani committed against our members, our fallen on 9/11 and our New York City union officers following that horrific day."
Ielpi, for his part, said he was "deeply disappointed and disheartened" by the union's recent political activities and called the letter offensive and inaccurate.
Tim Brown, a former firefighter and executive director of Firefighters for Rudy who is also a Giuliani campaign aide, added: "We are honored by the support of so many first responders from across the country and are appreciative of their continued enthusiasm for Mayor Giuliani's candidacy."
The union says it's bipartisan. It endorsed Democratic presidential nominee John Kerry in 2004.
At least 10 Republican and Democratic candidates plan to attend Wednesday's forum, including Democratic Sens. Hillary Rodham Clinton and Barack Obama, and former Sen. John Edwards. On the Republican side, the only top tier candidate who has committed is GOP Sen. John McCain of Arizona. Mitt Romney, the former Massachusetts governor, declined an invitation.