Manchester – With their presidential candidacies largely depending on a first-place finish in Tuesday's New Hampshire primary, Republicans Mitt Romney and John McCain clashed on illegal immigration in last night's debate at St. Anselm College.

Romney, the former Massachusetts governor who has seen his once-commanding lead in New Hampshire disintegrate, also engaged in sharp exchanges with former Arkansas Gov. Mike Huckabee, who was to looking to parlay last week's victory in the Iowas caucuses into traction in the nation's first primary.

The debate -- sponsored by ABC News, WMUR television and Facebook -- also included former New York Mayor Rudy Giuliani, former Tennessee Sen. Fred Thompson and Texas congressman Ron Paul.

On immigration

Questioned about an issue that he had admitted hurt him politically earlier in the campaign, McCain said he has not changed his basic beliefs on illegal immigration -- comments that drew fire from Romney.

"The fact is, the American people have lost trust in government, and we have to secure the border," the Arizona senator said, adding he continues to believe in a "fool-proof" temporary worker program.

"These are God's children," said McCain, insisting that he does not favor amnesty. "We have to address this issue as humanely and compassionately as possible but we have to protect our national security interests."

Romney said, "If you have 12 million people here illegally," and they are allowed to remain, "in fact, you have amnesty."

He said that ideally, all illegal immigrants should be effectively evicted from the country, although he said he realized that's not possible.

McCain said that, two years ago, Romney deemed the senator's position on immigration "reasonable" and did not amount to amnesty.

"That's not true," said Romney.

"You could look it up," responded McCain.

McCain said his plan mandates that illegals learn English and "then go to the back of the line like everyone else," then turned again to Romney.

It was inaccurate "for you to describe it as you do in the attack ads, my friend," he said. "You can spend your whole fortune on these attack ads, but it still won't be true."

Romney in turn, accused McCain of a personal attack.

McCain said his plan offers no special favors, but added he "would not tell a soldier fighting in Iraq that he was throwing his grandmother out of the country."

Giuliani said illegal immigrants should "come forward and get ID cards." He said those who do not should be thrown out and cited a precedent set by a Republican icon.

"Ronald Reagan did amnesty. I think he'd be in one of Mitt's negative commercials. Ronald Reagan gave amnesty. Straight-out amnesty," Giuliani said.

Responding to criticism from Thompson, Giuliani insisted, "New York City was not a sanctuary city" while he was mayor, although he did acknowledge granting amnesty for children needing education, for those seeking hospital treatment and those who report crimes.

Paul said he worried about Giuliani's call for an identification card for immigrants because, he said, it is "opening the door for a national ID card." Hostility toward illegal immigrants is generated by the economy putting pressure on the middle-class, he added.

On Iraq

McCain later slapped Romney with a veiled accusation that he has flip-flopped on several key issues.

"We disagree on a lot of different issues," said McCain. "But I agree with you that you are the candidate of change."

Romney responded, "The continued personal barbs are interesting but unnecessary."

Earlier, on questions about the Iraq war, Huckabee was on the attack against Romney, accusing him of flip-flopping on support for the Bush surge.

Romney said he supported the surge "from the very beginning." Huckabee disagreed and said Romney had voiced support for a "timed withdrawal."

"Don't try to characterize my positions," said Romney.

"Which one?" asked Huckabee.

"You will help us if you talk about your policies and what you believe and let me talk about mine," Romney said.

Thompson said that while Bush's policies were not arrogant, they were "not perfect." In a global war against Islamic extremists, Thompson said, "We should only go in where we are able to."

Giuliani said Bush, while imperfect, "got the big decision of his presidency right when he put us on offense against Islamic terrorism." He credited Bush for the fact the nation has not been attacked since 9/11 and called for a dramatic increase in the size or the nation's military.

McCain agreed, saying, "America is not safe," but "America is safer" than it was prior to 9/11.

Romney said military strength must be coupled with use of "non-military resources" to help move the world of Islam toward modernity and toward moderation." He called for 100,000 more troops in the military. Giuliani called for 200,000 more.

On health care

Romney, who supported mandated health coverage for Massachusetts but has backed away from that position running for President, said mandates should be determined on a state-by-state basis, to which Thompson replied, "When did you come up with that?"

"If people can afford to buy it, either buy it or pay your own way," Romney said.

When other candidates criticized the pharmaceutical industry, Romney defended it.

"Don't turn this pharmaceutical companies into the big bad guys," he said, prompting McCain to interject, "Well, they are."

On energy policy

On energy policy, all candidates called for a new effort on alternative sources.

Paul said the price of oil is being pushed up by inflation.

McCain said the United States will be sending $400 billion a year to Islamic countries for oil, with some of that money inevitably winding up in the hands of terrorists.

Thompson said he opposes a windfall profits tax on oil companies, calling oil prices a "function of supply and demand."

"We're not a nation that regulates the profits or losses" of private business, he said.

The race to Tuesday

As they took the stage in the first of two pre-primary GOP face-offs on back-to-back nights, McCain and Romney were under pressure to finish first in the primary to maintain viability in the nomination battle. Both have invested huge amounts of time and resources in the Granite State.

The debate, dubbed "Fight Night" and sponsored by ABC News, WMUR television and Facebook, began just an hour after CNN and WMUR released a poll completed by the University of New Hampshire showing McCain leading Romney, 33 to 27 percent with 14 percent for former Giuliani and 11 percent for Huckabee.

Tonight, McCain, Romney, Giuliani, Huckabee and former Tennessee Sen. Fred Thompson are scheduled to make their final pre-primary joint appearance in a forum sponsored by Fox News. Neither Ron Paul, who participated in last night's debate, nor fellow congressmen Duncan Hunter, who was excluded, were invited to take part in the Fox forum.