Republican Mitt Romney looked to get his White House campaign back on track on Tuesday, releasing two years of tax records hours after he battered rival Newt Gingrich in his most aggressive debate performance.

Romney, the former front-runner who was stung by Gingrich's 12-point victory in the South Carolina primary last week, bowed to political pressure and released records that showed he will pay $6.2 million (3.9 million pounds) in taxes on a total of $42.5 million in income for 2010 and 2011.

That was an effective tax rate of 13.9 percent in 2010 and an expected 15.4 percent rate in 2011, below the tax rate of most Americans because most of Romney's income came from capital gains that are taxed at a lower rate.

Gingrich, the former U.S. House of Representatives speaker, had hammered Romney in South Carolina over his refusal to release his taxes, sparking questions about Romney's vast wealth and his work as the former head of a private equity firm that critics say plundered companies and cut jobs.

With the race to find a Republican nominee to challenge President Barack Obama in the November election looking increasingly like a two-man fight, Romney attacked Gingrich in a Florida debate Monday night, calling him an unreliable leader who traded on his time in Washington to become an influence peddler.

The assault, by far the most aggressive launched by Romney during the Republican presidential campaign, was designed to halt growing momentum for Gingrich, who moved into the lead in two Florida polls released on Monday.

The speaker was given the opportunity to be the leader of our party in 1994, and at the end of four years, he had to resign in disgrace, Romney said.

'A TERRIBLE HISTORIAN'

Romney criticized Gingrich for his work for troubled mortgage giant Freddie Mac, his ethics troubles in Congress and for lobbying Republican lawmakers on health issues while getting paid by healthcare companies.

Much of Newt's momentum has been fueled by his debate performances, Republican strategist Todd Harris said. When Romney even debates him to a draw, it's got to be viewed as a good night for Romney.

Romney's campaign lined up two supporters - Representatives Connie Mack of Florida and Dave Camp of Michigan - to continue the attacks on Gingrich in a Tuesday conference call with reporters.

In Florida, where the housing crisis has been so severe, you have a candidate running for president who was lining his pockets from Freddie Mac and reporting directly to the lead lobbyist for Freddie Mac, Mack said.

Gingrich refused to engage Romney during many of his attacks in the debate, but again denied he had been a lobbyist for Freddie Mac and accused Romney of distorting his record.

He may have made a good financier. He's a terrible historian, he said of Romney.

The sustained attack by Romney made Monday's debate a sharp departure from the two in South Carolina, where Gingrich turned in strong performances that helped him overtake Romney. The Republican candidates, including former U.S. Senator Rick Santorum and U.S. Representative Ron Paul, will debate again on Thursday.

Hours before the debate Gingrich, under pressure from Romney, released his contract with Freddie Mac that showed he was hired by the office in charge of lobbying. The contract, for one year of the six he worked for Freddie Mac, showed the Gingrich Group was paid $300,000 for 2006.

Gingrich and Romney hoped releasing the documents would cut short the attacks over Freddie Mac and the tax records.

All of these issues ultimately become background noise. What matters is the way the candidates react to them, Harris said. Romney's issue with the tax returns was not the returns themselves, it was the fact people perceived he was uncomfortable talking about them.

A newly aggressive Romney has a hectic schedule on Tuesday as he tries to make up ground on Gingrich, including offering a prebuttal to Obama's State of the Union address.

Two new polls in Florida, which votes next Tuesday, showed Gingrich wiping out what had been a double-digit lead for Romney before the South Carolina result. A Rasmussen poll had Gingrich ahead by 9 percentage points, while an Insider Advantage poll gave him an 8-point edge.

(Writing by John Whitesides; Editing by Philip Barbara)