Republican presidential rivals Rudy Giuliani and Mitt Romney scornfully debated immigration Wednesday in a provocative CNN/YouTube debate in St. Petersburg, Florida, held just over a month before the first votes are cast.

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.

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.

Surprisingly, black issues got some attention from the presidential hopefuls as well, thanks to the videotaped queries from YouTube users. Giuliani and Huckabee took a question on the Republican Party's lack of support from the African-American community submitted by a Los Angeles resident who maintained that blacks "hold fairly conservative views." The subject of the propriety of publicly displaying the Confederate flag was addressed. And a father from Atlanta submitted a question, captured on video with his son, asking about the candidates' positions on inner city violence. "Two hundred to 400 black men die yearly in one city alone," he said. "What are you going to do about that war?"

First to respond, Romney lauded the questioner, saying his son "is pretty fortunate because he's got a dad standing next to him that apparently loves him by all appearances there, and that's probably the best thing you can do for a kid is to have a mom and a dad ... That's number one. And thank heavens Bill Cosby said it like it was. That's where the root of crime starts."

David McMillan from Los Angeles, in his question, noted that "on a variety of specific issues -- gay marriage, taxes, the death penalty, immigration, faith-based initiatives, school vouchers, school prayer -- many African-Americans hold fairly conservative views ... why don't we vote for you?"

"We probably haven't done a good enough job as a party in pointing out that our solutions, our philosophy, is really the philosophy that would be the most attractive to the overwhelming majority of people in the African-American and Hispanic community," replied Giuliani. "here are many, many issues on which we can reach out. I found that one of the best was moving people off welfare. I moved 640,000 people off welfare, most of them to jobs. I change the welfare agency into a job agency, and all of a sudden, I had people that had a future."

Former Arkansas Governor Mike Huckabee cited CNN's own poll results to bolster his claim that "some 48 percent of the African-Americans in my state did, in fact, vote for me," he said, noting that the figure "is unusually high for African-Americans voting for a Republican. Here's the reason why: Because I asked for their vote, and I didn't wait until October of the election year to do it ... I don't want to be a part of a Republican party that is a tiny, minute and ever decreasing party, but one that touches every American from top to bottom, regardless of race."

The racial politics of the Confederate flag came up via an inquiry from Houston resident Leroy Brooks, who asked candidates if "this flag right here represents the symbol of racism, a symbol of political ideology, a symbol of Southern heritage -- or, is it something completely different?"

Huckabee said that, "with the kinds of issues we got in this country, I'm not going to get involved with a flag like that ... The people of our country have decided not to fly that flag. I think that's the right thing."

He also used his answer to take a shot at Democratic candidate and former senator John Edwards.

"Every time I listen to someone like John Edwards get on TV and say there are two Americas, I just want to -- I just want to throw something at the TV, because there are not two Americas. There's one America," Huckabee said. "We are a nation united. We face extraordinary challenges right now. And Democrats dividing us and tearing down this country are doing exactly the wrong thing. We're succeeding in Iraq. We've got tough challenges. We can overcome them. But we do not need to have that kind of divisive talk. And that flag, frankly, is divisive, and it shouldn't be shown."

Former Tennessee Sen. Fred Thompson, noting that "everybody who hangs the flag up in their room like that is not racist," conceded that "I also know that for a great many Americans, it's a symbol of racism. As far as a public place is concerned, I am glad that people have made the decision not to display it as a prominent flag, symbolic of something, at a state capitol. As a part of a group of flags or something of that nature, you know, honoring various service people at different times in different parts of the country, I think that's different. But, as a nation, we don't need to go out of our way to be bringing up things that to certain people in our country that's bad for them."

At the outset of the debate, immigration dominated the questions submitted online and swept in the remainder of the Republican field.

Thompson 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.

Sen. John McCain, 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.

The most fierce exchanges were among the candidates with the most at stake with only five weeks left before the first voting in the presidential contest begins. Giuliani leads in national polls but trails Romney in early-voting Iowa and New Hampshire. Romney faces challenges from Huckabee in Iowa and from Giuliani and McCain in New Hampshire.

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 and whether New York taxpayers foot the bill for security he 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."

He didn't, however, offer an explanation for why the tens of thousands of dollars in costs, which aides say were routine expenses for protection for the mayor, were billed to city offices like the Office for People With Disabilities.

Tony Carbonetti, Giuliani's mayoral chief of staff and his top campaign political adviser, said he's asked Joe Lhota, a former city budget director, ex-deputy mayor and a Giuliani campaign adviser, to explain how such accounting practices could have occurred and why security expenses were not billed to the police department.

"These were all legitimate expenses incurred in protecting the mayor, and his police detail covered him wherever he went, 24/7," Carbonetti said in an interview before the debate. "You just do what you do, and the police go with you. That's just a fact of life when you're the mayor of New York."

Later, an aide said that for accounting purposes, the expenses appear to have been temporarily allocated to city offices and paid for out of the mayor's budget but that the police department ultimately picked up the tab and reimbursed the mayor's office at the end of each year.

Giuliani's affair with Nathan, while he was married to second wife Donna Hanover, has become common knowledge.

But the suggestion, true or not, that he was hiding expenses for liaisons with Nathan in little-known city accounts, could open him up to criticism, remind voters of his three marriages and infidelity and tarnish his good-guy image from the aftermath of the Sept. 11 terrorist attacks. The report surfaced just hours before debate.

None of Giuliani's rivals raised the issue.

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 Ron Paul, 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.