A combative Mitt Romney on Monday broadened his call for Republican presidential challenger Newt Gingrich to release records from his work as a consultant, speculating that those documents and records from a House ethics investigation during his time as speaker could show “potentially wrongful activity of some kind.”

“We could see an October surprise a day from Newt Gingrich,” the former Massachusetts governor told reporters at a news conference here. “And so let’s see the records from the ethics investigation, let’s see what they show. Let’s see who his clients were at the time he was lobbying Republican congressmen for Medicare Part D.

“Was he working or were his entities working with any health-care companies that could’ve benefited from that? That could represent not just evidence of lobbying but potentially wrongful activity of some kind?”

Romney — who has come under fire for not releasing his tax returns but who promised to do so Tuesday — called on his surging rival to return the money he was paid by mortgage giant Freddie Mac. He offered no evidence that he knew of wrongful activity by Gingrich. Nor did his advisers when pressed by reporters. The allegation was the latest sign that Romney is searching for a way to blunt Gingrich’s momentum, coming after his surprise win Saturday in the South Carolina primary.

Gingrich responded swiftly, saying he would release his contract with Freddie Mac and calling his rival a desperate candidate who will say almost anything.

“I have been told by a variety of people that Governor Romney has been saying unkind things. I prefer personally not to believe it,” Gingrich said to a crowd of about 150 people who gathered in front of the River Church in Tampa. “It’s such baloney. It used to be pious baloney, now it’s just desperate baloney. That’s the succession of this campaign, we’ve moved from Romney’s pious baloney to Romney’s desperate baloney.”

Before a candidate debate Monday night, Gingrich said that he had been memorizing the line “There you go again,” which Ronald Reagan used when squaring off against President Jimmy Carter in 1980.

“I’m thinking maybe I’ll finally convince him I really am a Reaganite if I just use President Reagan’s line,” Gingrich said.

Romney hopes to put Gingrich on the defensive heading into an intense week of campaigning before the Florida primary on Jan. 31. With his onetime momentum now gone, Romney is counting on his superior organization and financial advantage to gain an edge in Florida, the fourth-most-populous state in the nation and one with expensive media markets.

Romney has a head start on the airwaves. His super PAC, Restore Our Future, has spent $5 million in Florida, according to Federal Election Commission filings. And his campaign has been on the air for three weeks, running about the same number of ads as the super PAC, according to Kantar Media/CMAG.

Gingrich is working overtime to catch up. On the heels of his victory in the South Carolina primary on Saturday, his campaign set a goal of $1 million for an online “money bomb” but quickly raised it to $2 million over two days. And Winning Our Future, a PAC backing Gingrich, set a goal of spending $10 million on the Florida primary.

Miriam Adelson — the wife of Sheldon Adelson, a casino and hotel mogul whose $5 million donation helped make Gingrich competitive in South Carolina — has promised to give the Gingrich super PAC another $5 million this week, a source close to Adelson said.

The new pledge was first reported by Jon Ralston, a longtime Las Vegas Sun columnist who covers the gambling industry. Sheldon Adelson is the chief executive and primary owner of Las Vegas Sands, which owns the Sands Expo and Convention Center and Venetian hotel and casino complex in Las Vegas, and casinos and resorts in Pennsylvania, China, Macau and Singapore.

The Adelsons are longtime friends of the former House speaker and share his support for Israel. Miriam Adelson was born in Israel, and Sheldon Adelson is the son of Lithuanian Jewish immigrants.

Gingrich is also receiving a boost from an unexpected source: the American Federation of State, County and Municipal Employees, a union allied with President Obama. The union has spent $1 million on an ad linking Romney to Florida’s unpopular governor, Rick Scott (R).

In a state with extreme housing problems, Romney hit Gingrich hard over his work for Freddie Mac.

“He said in a debate, actually, that people who profited from the failed model of Freddie Mac and Fannie Mae ought to give back their money,” Romney said. “Well, the speaker made $1.7 million in his enterprises from providing services to Freddie Mac. He ought to give it back.”

Campaign officials said Romney will air a television advertisement in Florida this week that attacks Gingrich for his Freddie Mac connection. Titled “Florida Families,” the spot begins with a narrator saying, “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.”

The attack touches on a key vulnerability Gingrich has nationally among Republicans, who have grown increasingly negative about his work since he left elective office. Although most approve of the job he did as House speaker, the latest Washington Post-ABC News poll found that 51 percent of Republicans and GOP-leaning independents expressed unfavorable views of Gingrich’s work as a consultant for companies with interests in federal policymaking.

Romney is trying to regain ground in the volatile GOP nominating contest after watching his 23-point lead in national Gallup tracking polls a week ago shrink to a five-point edge over Gingrich among Republican-leaning registered voters. Thirty percent of respondents in the latest national surveys said they would vote for the former Massachusetts governor; 25 percent said they would back Gingrich.

Gingrich repeatedly has said that he never lobbied lawmakers on behalf of Freddie Mac and health-care companies, saying he was a consultant and a historian.

“I was not a lobbyist, I was never a lobbyist, I never did any lobbying. Don’t try to mix these things up. That fact is, I was an adviser strategically,” he said Sunday on NBC’s “Meet the Press.”

On Monday afternoon, the Gingrich campaign deployed former congressman J.C. Watts, a key lobbyist in rewriting federal regulations overseeing Freddie Mac, to defend the former speaker.

Watts said Gingrich was never a part of the discussions on Capitol Hill about Freddie Mac.

“In the six years that I was chairman of FM Policy focus, we had Republican and Democrat consultants that were part of our meetings,” Watts said. “We talked to Republican and Democrat members of Congress. We talked to Wall Street types. And the first time I heard Newt Gingrich’s name was probably 45 days ago.”

Former Minnesota governor and Romney surrogate Tim Pawlenty, on a conference call with reporters, said “the notion that he was paid $1.7 million as a historian for Freddie Mac is just BS.”

“Newt Gingrich has represented hundreds of clients and interest groups in Washington, D.C., since he left the speakership,” Pawlenty said. “To say that he wasn’t a lobbyist is incredible hairsplitting.”

Romney dismissed Gingrich’s contention that his work for corporate clients did not fall within the legal definition of being a lobbyist.

“If you’re working for a company, getting paid for a company through one of your many entities and then you’re speaking with congressmen in a way that would help that company, that’s lobbying,” Romney said. “If it walks like a duck, quacks like a duck, it is a duck.”

Romney also accused Gingrich of being a “highly erratic” leader through the course of his career.

He noted that Gingrich voted in favor of establishing the Department of Education, yet now says the department should be eliminated and its authority sent to the states. And Romney said Gingrich is “opposed vehemently” to the Massachusetts health-care system “and yet just a couple of years ago wrote about what a superb system it was.”

Romney added: “He’s gone from pillar to post almost like a pinball machine, from item to item in a way which is highly erratic and does not suggest a stable, thoughtful course which is normally associated with leadership.”

Staff writers Tim Farnam and Amy Gardner in Washington contributed to this report.