Newt Gingrich won a big victory in the South Carolina presidential primary Saturday with a populist message, attacking Washington elites (including the news media) and questioning Mitt Romney’s business practices.

Now, Romney hopes to turn the tables on Gingrich in Florida by focusing on what may be the House speaker’s biggest vulnerability: his post-Congress “consulting” career in Washington, D.C., particularly his work for Freddie Mac.

Freddie Mac and Fannie Mae are two congressionally chartered corporations that buy mortgages from lenders, providing these lenders with capital to make more home loans. The system looked like it was working until these two enterprises overdosed on subprime mortgages. When the housing market tanked, Freddie Mac and Fannie Mae became insolvent, and the federal government was forced to take them over.

Many Republicans blame Freddie Mac and Fannie Mae for the financial crisis, contending they encouraged lenders to make loans to buyers who eventually couldn’t afford them, all in the name of boosting home ownership rates in the U.S.

A firm founded by Gingrich, the Center for Health Transformation, received more than $1.6 million in fees over eight years from Freddie Mac. Gingrich said he didn’t do lobbying for Freddie Mac—he just provided advice to them as “a historian.”

That “is just bs—it’s just nonsense,” said Romney supporter Tim Pawlenty, a former governor of Minnesota whose own presidential candidacy fizzled out last year.

Pawlenty conducted a press call today focusing on the need for Gingrich to tell voters exactly what he did for Freddie Mac. Gingrich also should direct his former firm to release its contract with the government-sponsored enterprise and provide details on the work he and his former firm did for other clients in Washington.

“We have very little insight and transparency on what he did for many of those clients,” Pawlenty said.

Gingrich has been demanding that Romney release his tax records, and Romney now says he will do so on Tuesday.

But “transparency goes both ways,” says Will Weatherford, speaker designate of the Florida House of Representatives, who joined Pawlenty on the Romney campaign call.

Florida is ground zero for the foreclosure crisis, and average home values are down by 35 percent or more, Weatherford noted. Given the pain the housing market’s collapse has caused Floridians, Gingrich is going to have to answer questions about what advice he gave Freddie Mac and whether he advocated on behalf of the enterprise with members of Congress, Weatherford said.

“I don’t think Gingrich will be able to sweep this issue under the rug,” Weatherford said.

The Romney campaign began running a television commercial in Florida today that focuses on Gingrich’s work for Freddie Mac. Here’s the ad’s main message: “While Florida families lost everything in the housing crisis, Newt Gingrich cashed in.”

Gingrich today once again denied he did any lobbying work for Freddie Mac. Appearing on ABC’s Good Morning America, Gingrich said Romney is “deliberately saying things he knows are false.”

But he didn’t detail what he did do, and Pawlenty said for Gingrich to say he wasn’t a lobbyist is “incredible hairsplitting.”

New polls show that Gingrich is leading in Florida, which holds its primary on January 31. Now that he, and not Romney, is the frontrunner, Gingrich can expect a steady drumbeat of demands for him to release details of his work for Freddie Mac and other special interests in Washington. Romney’s hemming and hawing over his tax-records issue hurt him in South Carolina—will Freddie Mac kill Gingrich’s chances in Florida?

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Kent Hoover is the Washington bureau chief for bizjournals.