Washington (CNN) -- Mitt Romney ramped up his criticism of Newt Gingrich on Monday, prompting the former House speaker to respond that his chief rival for the Republican presidential nomination was getting desperate.
Surging in the polls after his double-digit victory in South Carolina last week, Gingrich said that presumptive front-runner Romney was seeing his early lead disappearing in Florida, the site of the next primary on January 31.
A new national poll Monday put Gingrich and the former Massachusetts governor in a statistical tie at the top, with former Pennsylvania Sen. Rick Santorum and Texas Rep. Ron Paul lagging behind. Gingrich also has narrowed Romney's lead among Florida voters.
All four candidates will participate Monday night in the first of two debates this week in Florida.
"If you've been campaigning for six years and you begin to see it slip away, you get desperate," Gingrich said Monday in Tampa. "And when you get desperate, you say almost anything, and I think at tonight's debate he'll stretch the barrier on almost anything."
Meanwhile, Romney called Gingrich a Washington insider lobbyist, questioned his leadership and demanded that he release records tied to a previous ethics investigation and work done for housing giant Freddie Mac.
Romney also demanded Gingrich return roughly $1.6 million earned from a contract with Freddie Mac and ridiculed Gingrich's insistence that the work amounted to little more than "strategic" advice, as opposed to lobbying.
"Saying that New Gingrich is a lobbyist is just a matter of fact. ... If it walks like a duck and quacks like a duck, it is a duck," Romney told reporters in Tampa. "We could see an October surprise a day from Newt Gingrich. Let's see the records."
Romney was expected to continue his new rhetorical offensive in the Monday night debate and throughout the run-up to the Sunshine State primary.
"As you look at the speaker's record over time, it has been highly erratic," Romney said. "He voted in favor of establishing the Department of Education, and yet he gets in a debate and says we should get rid of the Department of Education and send all the education issues back to the states. He 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."
Earlier in the day, Gingrich said he had asked his former company, the Center for Health Transformation, to release the details of its consulting contract with Freddie Mac. The company said it would release the contract and a statement on Monday evening.
Gingrich repeated his insistence that he did no lobbying of any kind.
"Romney keeps using the word lobbyist, I'm sure, because his consultants tell him it scores well," Gingrich said. "It's not true. He knows it's not true. He's deliberately saying things he knows are false."
Gingrich also accused Romney of hypocrisy.
"Here's somebody who has released none of his business records, who has decided to make a stand on transparency without being transparent," Gingrich said.
On Sunday, Romney yielded to mounting pressure to release his previous tax returns, promising to make public his 2010 return and an estimate of his 2011 tax liability.
It was a change from previous pronouncements by Romney that he would release his tax returns in April. Last week, Romney estimated his actual tax rate was close to 15% -- the amount charged for capital gains income -- because most of his income was from investments.
A wealthy former venture capitalist, Romney acknowledged on Sunday that he had mishandled the issue of releasing his tax returns.
"I think we just made a mistake in holding off as long as we did. It was just a distraction," Romney said during an appearance on "Fox News Sunday."
The latest maneuvering occurred against the backdrop of a GOP nomination fight that has changed dramatically over the course of the past week.
Initially announced as the victor in the January 3 Iowa caucuses by eight votes over former Santorum, Romney learned Thursday that certified results showed Santorum actually won the state by 34 votes. Then came Gingrich's solid victory Saturday in South Carolina, in a race that Romney had led until his uncertain handling of the tax return issue and a pair of debates widely considered to have been won dominated by the former speaker.
For his part, Gingrich on Sunday dismissed Romney's continuing critique of Gingrich's previous ethics controversy. Appearing on CNN's "State of the Union," Gingrich characterized a $300,000 penalty leveled by the House Ethics Committee in the late 1990s as reimbursement for the cost of the investigation.
He also claimed that he persuaded fellow Republicans in the House of Representatives to vote "yes" on the ethics charges against him to put a swift end to the proceedings.
"I personally asked House Republicans to vote 'yes' because we had to get it behind us to get back to the things that mattered," Gingrich said.
According to the nonpartisan fact check group PolitiFact, Gingrich was reprimanded by the House and ordered to pay the $300,000 penalty in 1997 for violating an ethics rule. It noted that the penalty was indeed considered reimbursement for the investigation.
The violation originated in a course Gingrich taught at Kennesaw State College, which organizers claimed qualified for tax-exempt status, PolitiFact reported. The House Ethics Committee ultimately concluded the course was run to "help in achieving a partisan, political goal," making it ineligible for tax exemption, according to PolitiFact.
Central to the 1997 investigation was a letter submitted by Gingrich's lawyers, which the ethics panel deemed inaccurate. Gingrich conceded Sunday the letter was a mistake.
"It was a mistake," Gingrich said Sunday. "So the one mistake I made was signing a letter written by our lawyers, a firm which frankly did me a great disservice. And that's the only thing."
CNN's Kevin Liptak, Ashley Killough, Kevin Bohn, Paul Steinhauser and Gregory Wallace contributed to this report.