ST. PETERSBURG, Florida (CNN) -- Fred Thompson aired the first negative television ad in the 2008 Republican presidential campaign race, using the CNN/YouTube debate Wednesday to deliver a double broadside against Mitt Romney and Mike Huckabee.
Fred Thompson's ad attacked Mitt Romney on abortion and Mike Huckabee on taxes.
With five weeks remaining before the Iowa caucuses, Thompson's video took aim at Romney for changing his position on abortion rights and Huckabee for his past statements on taxes.
Each of the eight GOP candidates was allowed to air a 30-second, YouTube-style campaign commercial during the two-hour debate.
The candidates' criticism of one another is not new -- they have been hammering each other for months on the campaign trail. But Thompson took it to a new level by doing so in this nationally televised debate.
The fireworks began well before the ad from the actor and former Tennessee senator was shown, as the pleasantries between the candidates quickly dissolved into a good old-fashioned street fight.
The Republican White House hopefuls took off the gloves as front-runners Romney, the former Massachusetts governor, and former New York Mayor Rudy Giuliani clashed over the issue of illegal immigrants. Watch the debate over immigration »
Thompson's video showed Romney talking about his support for Roe v. Wade, the Supreme Court decision that legalized abortion, during his failed 1994 Senate bid, and Huckabee expressing a willingness to go along with tax increases when he served as Arkansas governor.
"What were some saying during the Republican Revolution" flashed on the screen as viewers watched a clip of Romney saying: "I believe that abortion should be safe and legal in this country. I believe that since Roe v. Wade has been the law for 20 years that we should sustain and support it." Watch the questions and the candidates' responses »
The image then transitioned to Huckabee saying, "Others have suggested a surcharge on the income tax. That's acceptable. I'm fine with that. Others have suggested perhaps a sales tax. That's fine."
A closing quote then appeared on the screen, "We will win next November by sticking to our conservative principles."
Thompson defended the sharp tone of the ad, first with a little bit of humor and then by a simple blunt statement.
"I just wanted to give my buddies here a little extra airtime," Thompson said.
He then added, "These are their words."
Despite the potential explosiveness of the ad, Romney did not appear shaken by it and offered this mea culpa: "On abortion I was wrong."
"If people in this country are looking for someone who's never made a mistake on a policy issue and is not willing to admit they're ever wrong, they're going to have to find somebody else," he said.
And he noted that he changed his mind well before he decided to run for president.
"I'm proud to be pro-life," Romney said. "I'm not going to be apologizing to people for becoming pro-life."
Huckabee, too, did not allow the ad to knock him off stride.
"Well, I was governor nearly 11 years, and in that time I cut 90 taxes," he said. "Over that period of time, the income tax remained exactly what it was. The sales tax is one penny higher. But I did do a number of tax cuts that helped a lot of people all over the place, like eliminating the marriage penalty, doubling the child care tax credit, getting rid of capital gains on the sale of a home, cutting capital gains on other things."
In their argument over whose immigration record was worse, Giuliani accused Romney of operating a "sanctuary mansion" in reference to illegal immigrants that were found working at his home. Romney pointed out that it was a landscaping company, not him, who hired the workers. He then turned the tables on Giuliani.
"If you hear someone with a funny accent, you, as a homeowner, are supposed to go out there and say, 'I want to see your papers.'"
He added, "Is that what you're suggesting?"
The exchange between Romney and Giuliani set the tone for the night in what is likely to turn into a very bitter battle for the Republican nomination. The candidates now turn their focus to the early voting states of Iowa, New Hampshire, Michigan, Nevada and South Carolina.
Expect more punches to be thrown before the candidates return to Florida for that state's primary on January 29. E-mail to a friend