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 ." (Read more analysis from Ververs in Horserace.)
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.