Mitt Romney has ripped into Republican rival Newt Gingrich attacking what he said was the former speaker of the House of Representatives' disgraced exit and subsequent work as a Washington influence peddler.
But attention focused on Mr Romney's finances - including a now-closed Swiss bank account - after his campaign released tax returns in the early hours of Tuesday.
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 Mr Gingrich on the defensive a week before the Florida primary election on January 31. 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 Mr Gingrich leading Mr Romney after his decisive win in South Carolina over the weekend, making Florida pivotal if Mr Romney is to reassert his former role as the inevitable Republican nominee.
Mr Gingrich told Fox News that Mr Romney is a "desperate guy" throwing wild punches.
In the unsettled political atmosphere, Mr Obama will deliver his third State of the Union address 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 Mr Obama hopes for a second term.
The Republican leading candidates are wrestling with more personal economic issues. Both have now released documentation about issues that have bedevilled their campaigns.
Mr Romney, whose wealth is estimated to be as much as 250 million dollars (£160 million), put out income tax returns for 2010 overnight that show he is among the top 1% of US taxpayers. His income puts him in the top 0.006% of Americans.
Mr Romney's 2010 returns and estimates for last year also showed he paid 14% and 15.4% tax on the vast fortune he accumulated as a venture capitalist. That rate is far lower than standard rates for high-income earners whose income is derived from their wages rather than investment earnings.
He is seen by many as a member of America's superwealthy investor class that uses the US tax laws to pay a lesser rate than working people. He had refused until recently to disclose any tax returns, then hinted he would offer a single year's return in April. But mounting criticism and the South Carolina primary forced his hand.
He earned 21.7 million dollars income in 2010, most of it from his investments, and for 2011 he will pay about 3.2 million dollars, his campaign said.
As his newly released returns were studied, Mr Romney's advisers acknowledged that he once had a Swiss bank account but had it closed in 2010 as he prepared to enter the race for the White House.
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.