(CNN) -- GOP presidential front-runners Mitt Romney and Newt Gingrich barnstormed across Florida on Friday, trying to rally supporters and break away in the polls four days before what is shaping up to be a pivotal primary in that state.
The morning after a heated CNN/Florida Republican Party debate in Jacksonville, a pro-Gingrich super PAC hit the airwaves with a television ad essentially accusing Romney of committing Medicare fraud. One of Gingrich's top Florida backers, former state Attorney General Bill McCollum, also raised questions about Romney's past service on the board of Damon Corp., a Massachusetts medical testing company that pleaded guilty in 1996 to billing Medicare fraudulently and paid a $119 million fine.
Romney was never implicated in any wrongdoing, but the issue is "fair game" and "raises a serious question about (Romney's) corporate judgment," McCollum said.
McCollum said he does not believe Romney was guilty of fraud while sitting on the Damon board. "But did he perform the function he should have performed? Or was he negligent in that function? ... That's the issue there," McCollum said.
The Romney campaign denounced the super PAC ad, saying it "would make Michael Moore proud and (has) already struck a chord with President Obama's liberal allies."
"They have been thoroughly discredited by independent fact checkers and by respected Republicans like Jeb Bush and Marco Rubio," Romney spokeswoman Andrea Saul said. "Once again, Speaker Gingrich is reaching into President Obama's playbook and trying to re-litigate old Democrat attacks. Speaker Gingrich and his political cronies are desperate to distract from his record of failed and unreliable leadership, and voters won't be fooled."
Gingrich's campaign also released a new ad characterizing Romney as an untrustworthy, flip-flopping politician. The spot includes a clip from a Mike Huckabee attack ad against Romney in 2007, in which the former Arkansas governor said about his presidential-campaign opponent, "If a man's dishonest to get a job, he'll be dishonest on the job."
In a statement shortly after the ad emerged, Huckabee objected to the use of his old campaign ad for Gingrich's purposes, and said he's remaining neutral in the primary fight.
Meanwhile, both Romney and Gingrich stepped up their Latino outreach efforts Friday with speeches to prominent Hispanic groups in Miami. Romney's campaign trumpeted an endorsement from Puerto Rico Gov. Luis Fortuno, who campaigned with Romney Friday evening in Orlando.
Rick Santorum also stumped for votes in the Miami area. Speaking to CNN's Wolf Blitzer on "The Situation Room," the former Pennsylvania senator denied reports that he's on the verge of dropping out of the race.
"There hasn't even been a discussion ... as to whether to get out of this race," Santorum said, noting that his campaign is currently hiring staffers for states holding primaries in March and April.
For his part, Ron Paul headed up to Maine, which holds its GOP caucuses starting next week. The libertarian Texas congressman has all but conceded Florida's 50 delegates and is concentrating much of his effort on less expensive caucus states where his small but enthusiastic army of supporters can have a greater impact.
A new Quinnipiac University survey released Friday indicates Romney has opened up a lead of 9 percentage points over Gingrich. Romney has the support of 38% of likely Florida primary voters, according to the poll. Gingrich has the backing of 29% of likely voters, while 14% support Paul and 12% back Santorum.
The poll, conducted Tuesday through Thursday, has a sampling error of plus or minus 4.1 percentage points.
A number of political analysts said a win for Romney in Florida could all but seal the Republican nomination for the former governor. A Gingrich victory, on the other hand, would almost certainly set in motion a long, bitter primary struggle. It also would undermine any aura of inevitability surrounding Romney, who suffered a major defeat to Gingrich last week in South Carolina.
Needing a strong showing to try to blunt Gingrich's harsh attacks of recent days, Romney was forceful in Thursday night's debate, and had the former House speaker on his heels on several issues.
Among other things, Romney used Thursday night's debate to provide his most forceful and unapologetic statement so far about his vast personal wealth, saying Republicans such as Gingrich shouldn't criticize him for being successful.
The former governor -- who spent a number years as a venture capitalist -- released his tax returns this week under pressure from Gingrich and others, revealing he paid a rate of less than 15% on income in the millions, mostly from investments, including some holdings in offshore accounts.
On the campaign trail, Gingrich has disparaged Romney's wealth, saying he lives in a world of "Swiss bank and Cayman Island bank accounts" and did nothing for his money. Romney previously seemed uncomfortable discussing his finances -- initially stalling on releasing his tax records -- but was ready Thursday evening.
"I think it's important for people to make sure we don't castigate individuals who have been successful and try to suggest there's something wrong with being successful and having investments and having a return on those," Romney said.
Addressing Gingrich directly, he said: "You indicated that somehow I didn't earn that money."
"I have earned the money that I have," Romney continued. "I didn't inherit it. Those investments lead to jobs being created in America. I'm proud of being successful. I'm proud of being in the free enterprise system. I'm not going to run away from that."
Other debate exchanges were just as compelling.
Asked to address the housing crisis, one of the major problems facing Florida voters, Gingrich began by claiming that Romney was knowingly and "unfairly" attacking him on his consulting record for mortgage giant Freddie Mac, sparking a fiery back-and-forth over which candidate has had a closer relationship with troubled lenders.
Gingrich said Romney had profited off investments in both Fannie Mae and Freddie Mac. In turn, Romney explained his holdings were in a blind trust and involved mutual funds that included bonds of the mortgage lenders.
Then he turned the tables on Gingrich, pointing out that Gingrich also had investments involving Fannie Mae and Freddie Mac.
Later, though, Santorum launched the toughest attack of the night on Romney over the health care plan Romney passed in Massachusetts that in some ways served as model for the federal health care legislation under the Obama administration.
Positioning himself as the strongest opponent of the health care law, Santorum insisted both the Romney plan and the federal one included a mandate to own health insurance or face a fine -- a concept detested by conservatives.
Santorum repeatedly returned to that point when Romney tried to argue the difference between a plan for Massachusetts and a federal plan for the country, saying at one point: "Your mandate is no different than Barack Obama's mandate."
Rachel Streitfeld, Peter Hamby, Adam Aigner-Treworgy and Tom Cohen contributed to this report.