The Republican debate in Tampa, Florida, has toughened up Mitt Romney in his fight against Newt Gingrich as he takes him seriously as a rival after Gingrich's surprise win in the South Carolina Primary.

Romney, the former Massachusetts governor, and his camp are trying to thwart any chances of a win by Newt Gingrich in the Florida primary on 31 January.

Though the debate covered topics such as illegal migration, sugar subsidies, Nasa funding and Terry Schiavo, Mitt Romney focused more on Gingrich's past as an adviser to mortgage lender Freddie Mac.

Gingrich, the former speaker of the House, was reportedly on a contract with the mortgage giant which earned him $1.6m (£1m).

"This is the first time in American history that a speaker of the House has had to resign in disgrace. We can't possibly retake the White House if the candidate was working for the head lobbyist of Freddie Mac," the Guardian reported Romney as saying in the debate hosted by NBC's Brain Williams.

"I think it's pretty clear to say I never did any lobbying," responded Gingrich.

The Romney camp has also come out with an advertisement on Gingrich's association with Freddie Mac in Florida.

It says: "While Florida families lost everything in the housing crisis, Newt Gingrich cashed in. Gingrich was paid over $1.6 million by the scandal-ridden agency that helped create the crisis."

Gingrich was quick to deny the allegation calling it Romney's "misinformation" and a "false charge."

Though stories about Gingrich's marital infidelities and "unethical behaviour" are hotly debated, opinion polls still give the former House speaker an edge over Romney.