WASHINGTON — Senator McCain of Arizona is fending off attacks from his fellow Republican presidential candidates for his backing of a bill that would grant legal status to millions of illegal immigrants.

The issue towered over the Republican debate in New Hampshire last night, as Mayor Giuliani called the compromise legislation now before Congress “a typical Washington mess” while other hopefuls termed it “disastrous” and “dangerous.”

While acknowledging that the legislation “is not the bill I would have written,” Mr. McCain nonetheless defended it vociferously, and he challenged his critics to come up with better ideas.

“We’ve done exactly what you expect us to do, my friends, and that is come together,” the Arizona senator said, describing the coalition of Republicans, Democrats and the president that support the bill. “We need to act, my friends.”

Mr. Giuliani criticized the bill for lacking a “unifying purpose,” which he said should be a uniform way to find out precisely who is entering and leaving the country.

“The litmus test we should have for legislation is, is it going to make things better?” he said. “When you look at these compromises, it is quite possibly going to make things worse.”

The assault, and Mr. Giuliani’s description of what he would want to see in an immigration bill, elicited a jab from Mr. McCain, who has largely refrained from criticizing the former mayor during the campaign thus far. “Rudy, you just described our legislation, so I’d be glad to have further conversations with you, because it does account for people who are here illegally, it does have an employment verification system, and it gives others a chance to remain in this country,” he said, describing in broad terms elements of immigration reform that Mr. Giuliani has previously called for. Mr. Giuliani, standing next to him on the stage, smiled and shook his head.

The mayor retorted that he had “read the 400 pages” of the bill, and he pointed out technical elements that he didn’t see included. “This is part of the problem in Washington,” he said. “They say things, and then they’re not in the legislation.”

Despite the sharp, if cordial, exchange between Messrs. McCain and Giuliani, growing tensions between the Arizona senator and another top rival, Mitt Romney, appeared to cool off a day after Mr. McCain accused the former Massachusetts governor of pandering for votes on the issue. Asked to respond to Mr. McCain, Mr. Romney sought the high road. “He’s my friend. He campaigned for me two times,” Mr. Romney said. “I’m not going to make this about personal politics. This is too important an issue for that.”

The focus on immigration last night allowed several long shot candidates to join the fray. A congressman from Southern California, Rep. Duncan Hunter, advocated a larger, double-lined fence on the border running higher and hundreds of miles longer than the one currently being built. “If they get across my fence, we sign them up for the Olympics,” Mr. Hunter said.

A Colorado congressman, Rep. Tom Tancredo, who is running on an anti-immigrant platform, railed against the increased prevalence of the Spanish language in America. “We are becoming a bilingual country, and that is not good,” he said.

Mr. Romney was asked to defend his Spanish-language campaign ads and spoke about wanting to welcome everyone. Mr. McCain spoke of immigrants who served in the military, some of them not even American citizens.

The debate was the third featuring the same 10 Republican candidates. It was broadcast live on CNN from St. Anselm College in Manchester, the same location where two nights earlier the eight declared Democratic candidates sparred for two hours.

The forum was notable for an unusual number of questions about religion and social issues. At one point a former Arkansas governor, Michael Huckabee, a former pastor, noted that he was getting “all the moral questions.” He joked that it was better than getting all the “immoral” queries.

Mr. Giuliani took advantage of a lightning storm occurring during the debate, causing a technical glitch that helped him get through a potentially awkward question. CNN moderator Wolf Blitzer asked him to respond to a Catholic bishop in Rhode Island who lambasted Mr. Giuliani position on abortion and compared him to Pontius Pilate. As he began to answer, his microphone crackled loudly, indicating that lightning had struck overhead. Drawing a roar of laughter, Mr. Giuliani — along with Messrs. Romney and McCain standing next to him — backed off and looked up in the direction of the sky. “For someone who went to parochial schools all his life, this is something very frightening happening to me right now,” he joked, eliciting more laughter.

The debate also featured several gibes at President Bush. A former Wisconsin governor who served in his cabinet, Tommy Thompson, was asked how he would use Mr. Bush as a former president. “I certainly would not send him to the United Nations,” Mr. Thompson said, in a reference to Mr. Bush’s tense relations with the world body.

Mr. Tancredo went even farther, saying he wouldn’t even let Mr. Bush back in the White House after receiving a similar threat from the president’s political adviser, Karl Rove.