Giuliani, the front-runner in national polls, accused Romney of employing illegal immigrants at his home and running a "sanctuary mansion." The testy personal exchange came after Romney said Giuliani had retained New York's status as a sanctuary city while he was mayor.
Romney said it would "not be American" to check the papers of workers employed by a contractor simply because they have a "funny accent." He had landscapers at his Belmont, Mass., home who turned out to be in the country illegally.
Giuliani shot back, calling Romney's attitude "holier than thou."
"Mitt usually criticizes people when he usually has the far worse record," Giuliani said.
The audience, however, booed Giuliani as he tried to persist in his criticism of Romney.
"Romney is in many ways fighting a campaign on two fronts," said CBSNews.com Senior Political Editor Vaughn Ververs. "As the perceived front-runner in the first two primary states, he's getting squeezed in both and you saw that reflected in his exchanges tonight with Giuliani and Horserace.)." (Read more analysis from Ververs in
The confrontation came at the start of an innovative CNN-YouTube debate that forced the candidates to confront immigration immediately, signaling the volatility of the issue among Republican voters. The eight Republican candidates encountered a range of questions, including abortion, gun control from a gun wielding NRA member, and farm subsidies from a man eating an ear of corn.
They were even asked if they believed every word in the Bible by a man holding the holy book, and a question on the powers of the vice president from a gun-toting cartoon-version of Dick Cheney.
No one was exempt in the free-for-all as the candidates squabbled over interrogation techniques, the Iraq war, crime and who wields the most conservative record. The candidates tried to position themselves to the right of each other, knowing full well that conservatives hold sway in selecting the GOP nominee.
At the outset, immigration dominated the questions submitted online and swept in the remainder of the Republican field.
took the opportunity to distinguish himself from both Romney and Giuliani, arguing that Romney had supported President Bush's plan to provide a path to citizenship for some immigrants in the United States illegally now. He took Giuliani to task for attacking Romney's employment of illegal immigrants.
"I think we've all had people who we've hired who in retrospect was a bad decision," he said, alluding to Bernard Kerik, Giuliani's disgraced former police commissioner who is under federal indictment on multiple charges.
, for whom the immigration issue has proved particularly vexing, defended his support for an unsuccessful overhaul of immigration laws that included a temporary worker program and a path to citizenship.
"We must recognize these are God's children as well," McCain said. "They need our love and compassion, and I want to ensure that I will enforce the borders first. But we won't demagogue it."
Huckabee, who has also come under GOP criticism for some of his immigration policies while governor of Arkansas, defended benefits he supported for children of illegal immigrants, including allowing children to be eligible to apply for college scholarships.
"Are we going to say kids who are here illegally are going to get a special deal?" Romney asked.
Huckabee objected, saying the benefit was based on merit. "We are a better country than to punish children for what their parents did," he said.
Thompson, in what amounted to one of the first video attacks of the campaign, questioned the conservative credentials of two of his rivals in a YouTube clip. The video challenged Romney on abortion and Huckabee on taxes.
"I wanted to give my buddies here a little extra air time," Thompson said to laughter as he defended the video.
For Thompson, Romney and Huckabee are his biggest obstacles toward establishing himself as the candidate of conservatives.
"I was wrong, I was effectively pro-choice," said Romney, who has said he changed his stance in 2004 during debates on stem cell research. "On abortion, I was wrong."
"If people are looking for somebody in this country who has never made a mistake ... then they ought to find somebody else," he said.
As the front-runner, Giuliani faced questions about gun control, abortion andhe received while the married mayor visited his then-girlfriend, Judith Nathan, now his wife.
Giuliani said he had 24-hour protection as mayor because of threats against him and said all costs incurred were proper.
"I had nothing to do with the handling of their records," he said of how his security detail reported the expenses. "And they were handled, as far as I know, perfectly appropriately."
McCain, who has shown no love for Romney during the campaign, seized on Romney's response to a question about the legality of waterboarding as an interrogation technique. Romney said that as a candidate he would not publicly discuss what techniques he would rule out. That prompted McCain, a former Vietnam POW, to assert that waterboarding is indeed torture and should not be tolerated.
"Governor, let me tell you, if we're going to gain the high ground in this world ... we're not going to torture people," McCain said. "How in the world someone could think that that kind of thing could be inflicted on people who are in our custody is absolutely beyond me."
McCain also engaged, a Texas congressman whose libertarian views and opposition to the war have attracted thousands of donors, millions of dollars and a devoted online following.
McCain said Paul is promoting isolationism in calling for the United States to disengage from the war. "We allowed (Adolf) Hitler to come to power with that attitude of isolation," he said.
Paul objected, saying McCain had misunderstood his support for nonintervention with isolationism.
"I want to trade with people, talk with people, travel," Paul replied. "But I don't want to send troops overseas using force to tell them how to live." Later he made clear he would not run as an independent, despite demands from many of his supporters.
One questioner, retired Brig. Gen. Keith Kerr of Santa Rosa, Calif., asked the candidates about their views on gays in the military as he revealed himself to be gay.
Huckabee, Romney, McCain and Rep.all said they supported the current "don't ask, don't tell" policy.
The debate ended as it began, with Romney and Giuliani in a deeply personal dispute - over the New York Yankees vs. the Boston Red Sox.
"When I was mayor of New York City, the Yankees won four world championships," Giuliani said. "Since I've left being mayor of New York City, the Yankees have won none."
Romney replied: "Like most Americans, we love our sports teams and hate the Yankees."