WASHINGTON — Mayor Giuliani’s absence from a forum that included 10 presidential hopefuls is allowing senators Clinton and McCain an opening to try to seize the spotlight on an issue the former mayor has dominated early in the presidential campaign: the response to the terrorist attacks of September 11, 2001.

Addressing hundreds of firefighters at a presidential forum here yesterday, the two candidates from opposing parties profusely praised the work of first responders at the World Trade Center site while criticizing the federal government for a lack of preparedness and funding.

“You were there when we needed you, and I want you to know that I will be there with you when you need me,” Mrs. Clinton told members of the International Association of Fire Fighters, who heard from the candidates throughout the day.

Touching on an emerging campaign theme, Mrs. Clinton said that despite promises and rhetoric to the contrary, firefighters were among many Americans who are “invisible” to President Bush, citing inadequate health care and insufficient money to treat workers who became ill after September 11. “It’s great for the photo ops, but how about taking care of the people who have taken care of us across our country?” she said, drawing a standing ovation from the crowd.

Mr. McCain, a Republican of Arizona who has fallen behind Mr. Giuliani in the polls, quoted the September 11 commission report in taking aim at the defective radio communications that plagued firefighters who rushed into the trade center.

“The cause of this failure was not the first responders’ fault. It was the fault of the federal government,” Mr. McCain said.

Mr. Giuliani backed out of the forum amid a feud with union leaders, who are angry with him over his decision as mayor in November 2001 to reduce the number of firefighters searching for remains at ground zero. Leading in many national polls for the Republican nomination, Mr. Giuliani, through his image as “America’s mayor,” has become synonymous with the response to September 11 in the minds of many potential voters.

His campaign cited a scheduling conflict to explain his absence.

The memory of the attacks loomed over the forum yesterday — large photographs of ground zero draped the side of the hotel ballroom, and a video displaying images of the firefighters’ response played during breaks in the program. Without mentioning Mr. Giuliani, several candidates sought to establish their own connection to the day during their speeches.

A Republican former governor of Virginia, James Gilmore, described how he responded “decisively” to the attack on the Pentagon and promised that if he is elected president, “the radios are going to work.”

Yesterday’s forum, the first this year to feature candidates from both parties, also offered hopefuls an opportunity to court union support. A former senator of North Carolina, John Edwards, sought to set a high standard early on, saying he was proud to “walk a picket line,” effectively daring the other candidates to say the same.

While the audience responded warmly to Mr. Edwards, Mrs. Clinton drew much more enthusiastic responses than he or Senator Obama of Illinois did. She combined harsh criticism of the Bush administration with strong support for organized labor.

Throughout the day, kind words for unions elicited the loudest cheers from firefighters, and several said Mrs. Clinton struck the right tone. “I love her enthusiasm, her support,” a firefighter from Detroit, Verdine Pierce, said.

“She knows our issues,” the president of a firefighter union in Buffalo, Joseph Foley, said.

Among Democratic hopefuls, Senator Dodd of Connecticut and Senator Biden of Delaware also generated raucous cheers with their speeches, which featured lengthy praise of firefighters and unions in general. Governor Richardson of New Mexico sought to elevate his candidacy with humor and a few sharp jabs at his Democratic opponents, all of whom are current or former members of Congress. “I’m not going to say much about Congress — I have a real job. I do things,” Mr. Richardson, himself a former congressman, said to laughter. Referring to his standing in the polls, he added: “I’m at 4%, I’ve got to do something.”