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 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 US 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.