Republican presidential candidate Mitt Romney ripped into rival Newt Gingrich in the latest debate, attacking what he said was the former speaker of the House of Representatives' disgraced exit from that high post and subsequent work as a Washington influence peddler.

The leaders for the Republican nomination to challenge President Barack Obama in November appeared to have exchanged roles in the 18th debate, with the fiery Gingrich on the defensive a week before Florida votes on 31 January. The state's diversity, large population and large media markets are a bigger challenge than the three states that have voted in the nominating process so far.

Surveys show Gingrich leading Romney after his decisive win in South Carolina over the weekend, making Florida pivotal if Romney is to reassert his former role as the inevitable Republican nominee.

Gingrich told Fox News on Tuesday morning that Romney is a "desperate guy" throwing wild punches.

In the unsettled political atmosphere, Obama will deliver his third State of the Union address Tuesday night before a joint session of Congress. The nationally televised speech will be watched closely more for its political message in an election year than for policy initiatives as Obama hopes for a second term. Congress is virtually deadlocked, with Republicans holding the majority in the House and Democrats ascendant in the Senate, meaning Obama initiatives can be held hostage to politics.

Obama also remains vulnerable on the economy, the top issue in the election, and has low approval ratings for his handling of the slow rebound from the devastating 2007-2009 Great Recession.

The Republican leading candidates are wrestling with more personal economic issues.

Both Romney and Gingrich have now released documentation about issues that have bedeviled their campaigns. Romney put out income tax returns for 2010 and estimates for last year that showed he paid 14 per cent and 15.4 per cent government assessments on investments of the vast fortune he accumulated as a venture capitalist. Gingrich released a 2006 contract outlining his work as a consultant for the federally backed mortgage guarantor Freddie Mac.

Even as Gingrich has pulled even with or surpassed one-time front-runner Romney, both men have been saddled with questions about their financial and work records. Romney is seen by many as a member of America's superwealthy investor class that uses the US tax code to pay a lesser rate than working people.

Gingrich has fought off his past ethics reprimand while House speaker and the coup among members of his own party that led to his ouster from the leadership post and later resignation from Congress. He is also weighed down by charges that he worked as a lobbyist for Freddie Mac, an agency held up by some as responsible for the collapse of the US housing market that led to the near collapse of the US financial system in late 2008.

In Monday's debate, former Pennsylvania Sen. Rick Santorum and Texas Rep. Ron Paul were left mainly on the sidelines, watching Gingrich and Romney clash.

Romney predicted that his 2010 tax returns and an early report on the federal assessment for last year will generate talk but no surprises. There are continuing calls for him to release earlier tax returns from the period when he worked at Bain Capital, the venture firm, and to explain why he has some of his fortune parked in investments in the Cayman Islands, a location that serves many as a tax shelter.

"You'll see my income, how much taxes I've paid, how much I've paid to charity," he said. "Will it be an article? Yeah. But is it entirely legal and fair? Absolutely. I'm proud of the fact that I pay a lot of taxes."

Romney earned $21.7 million income in 2010, most of it from his investments, according to records his campaign released within hours of the debate. He donated a combined $3 million to charitable causes and the Mormon Church and paid about $3 million in federal income taxes for an effective tax rate of about 14 percent, the records showed. That rate is far lower than standard rates for high-income earners whose income is derived from their wages rather than investment earnings.

For 2011, Romney will pay about $3.2 million with an effective tax rate of about 15.4 per cent, the campaign said.

Gingrich, meanwhile, insisted his contract with Freddie Mac proves he was a consultant and not a lobbyist paid to influence congressional legislation that could affect the mortgage giant. Gingrich's consulting firm was paid $1.6 million for its work with the agency.

"You've been walking around the state saying things that are untrue," Gingrich told Romney.

Romney countered that Gingrich also had lobbied lawmakers to approve legislation creating a new prescription drug benefit under Medicare, the government program which provides health care coverage to the elderly.

"I have never, ever gone and done any lobbying," Gingrich retorted, adding that his firm had hired an expert to explain to employees "the bright line between what you can do as a citizen and what you do as a lobbyist."