With a Florida victory in his sights, Republican presidential frontrunner Mitt Romney tried to kill off primary rival Newt Gingrich's campaign unleashing a hard-hitting advert state-wide. Romney -- leading handily in the polls -- on Saturday sought to sweep away the largest obstacle to him winning the party nomination, hitting Gingrich over past ethics violations and dramatically calling into question his integrity. After a feisty debate performance on Thursday, Romney is in the ascendancy in Florida with just three days to go before Tuesday's primary. According to poll averages, the former governor of Massachusetts and multimillionaire venture capitalist leads in the sprawling Sunshine state with 39 percent against 31 percent for Gingrich, the former House of Representatives speaker. Former Pennsylvania senator Rick Santorum and Texas congressman Ron Paul trail the field with 11 percent and 10 percent, respectively. Time is running out for Gingrich to claw back some lost ground in a race that could prove pivotal in deciding who faces President Barack Obama in November. If Gingrich loses in Florida his path to the nomination is far from clear. He did receive an endorsement Saturday from a former rival for the nomination, pizza magnate Herman Cain, US media reported. "If we win on Thursday, I will become the Republican nominee," he said on the campaign stump in Orlando on Saturday, trying to rally support. "I am going to ask each of you to go out on Facebook, YouTube and Twitter, and email, and even by telephone, or talk to people face to face... I want to ask you reach out all your friends and neighbors." "We nominated a moderate in 96 and we lost. We nominated a moderate in 2008 and we lost, only a solid conservative can debate Barack Obama and win." But trying to seize on his rival's precarious position, Romney's ad uses television news footage from 1997 when Gingrich was reprimanded by a House ethics committee. It shows only iconic news anchor Tom Brokaw reporting events: "Newt Gingrich, who came to power, after all, preaching a higher standard in American politics, a man who brought down another speaker on ethics accusations, tonight he has on his own record the judgment of his peers, Democrat and Republican alike. "By an overwhelming vote, they found him guilty of ethics violations; they charged him a very large financial penalty, and they raised -- several of them -- raised serious questions about his future effectiveness." Gingrich was accused of dozens of violations, including a claim of tax-exempt status for his college course. A Romney campaign official refused to disclose the exact amount spent on the ad, but said "it's running state-wide, it's a heavy buy. It's a heavy buy." The Gingrich campaign described the ad as "another big lie" from Romney's team. "What the Romney campaign is hoping the American people don't remember is that in 1999, the IRS cleared speaker Gingrich of the substance of the ethics committee investigation," his campaign said. The two men barnstormed Florida in the closing days of a bitter race for the biggest state yet in the Republican nominating contest. On Saturday Romney sought to harden his conservative credentials by doling out a thumping to Obama in front of a Pensacola crowd peppered with military veterans, painting the president's foreign policy as buckling to US enemies. "He has the view that America is in decline and that the best course for America is to appease and accommodate the worst actors in the world," Romney said, seeking to convince those listening that he can be commander-in-chief. "The foreign policy of pretty please is not working terribly well," Romney said, citing threats from North Korea, Iran, Cuba and Venezuela. Flanked by Vietnam veteran and 2008's losing Republican presidential nominee John McCain, Romney vowed to increase the military by 100,000 -- instead of the Obama administration's proposal to cut it by that number to reduce costs after a decade of war in Iraq and Afghanistan. "I will make sure that our military is so strong that no one in the world would think of testing it," Romney said. Gingrich started the day in Port St. Lucie after releasing a new attack ad lambasting Romney's suitability for high office. "What kind of man would mislead, distort and deceive just to win an election?" the ad's narrator asked, alluding to Gingrich's assertion that Romney wrongly smeared him during their latest debate. Romney has touted his business experience as being better suited to the tasks of creating jobs and bringing strong growth to a lackluster US economy -- and has poured scorn on Gingrich's background as a Washington lawmaker. But Romney's own record in business has attracted challenges to his claim to have created more than 100,000 jobs, and he has been on the defensive over his immense wealth and low tax rates, revealed this week in the release of his 2010 tax return. Meanwhile, The New York Times reported that investment bank Goldman Sachs, which is managing much of Romney's $250-million fortune, has emerged as his largest single source of campaign money so far, with its employees having contributed at least $367,000.