US Republicans Mitt Romney and Newt Gingrich traded barbs in a national debate in the US state of Florida, ahead of the primary vote there.
The rivals clashed early on as Mr Romney called Mr Gingrich an "influence peddler", stating that he resigned from Congress in "disgrace".
Mr Gingrich shot back that the wealthy businessman "can't tell the truth".
In two polls released on Monday, Mr Gingrich's lead among likely Florida voters grew to as much as 9%.
Fellow candidates Rick Santorum, a former senator, and Texas Congressman Ron Paul also took to the stage on Monday.
Lots of taxes
In the fierce opening exchanges of the debate Mr Gingrich and Mr Romney sparred over the issue of their electability.
Mr Gingrich said that he would not "manage the decay" in Washington, while Mr Romney contrasted his leadership style with what he called Mr Gingrich's resignation from Congress in "disgrace".
Early on, Mr Romney admitted a tactical shift towards attacking his rivals after scrutiny over his tax returns sent his poll rates sliding ahead of the primary vote in South Carolina.
"I learned something from that last contest in South Carolina, and that was I had incoming from all directions, was overwhelmed with a lot of attacks. And I'm not going to sit back and get attacked day in and day out without returning fire," he said.
Meanwhile, Mr Gingrich refused to respond to some of the attacks and promised to create a website rebutting Mr Romney's criticism.
Mr Romney and Mr Gingrich clashed over the former Speaker's contract with the federal mortgage lender Freddie Mac, with Mr Romney saying Mr Gingrich was doing business with Washington's "chief lobbyists".
Mr Romney added that in Florida, where the housing market remains depressed, many people were suffering as a result of federal housing policy.
But Mr Gingrich countered: "I have never, ever gone and done any lobbying."
Mr Romney fought off suggestions that he was insufficiently conservative, citing his record as a Republican governor in a liberal state.
He was also forced to defend himself on the issue of his taxes and said that when he discloses his tax returns on Tuesday it would be clear that he pays "not a dollar more" than what he owes, adding that he pays "a lot of taxes".
The debate marked the first time the four presidential contenders appeared on a national stage after the exit of former Utah Governor Jon Huntsman and Texas Governor Rick Perry from the field.
Florida is a key battleground state with a large Latino population, a higher-than-average rate of unemployment and a large retired community.
Earlier on Monday Mr Gingrich released his 2006 contract with federal mortgage agency Freddie Mac amid pressure to explain the nature of his employment there.
Mr Romney had called his rival erratic like a "pinball machine", challenging him to return $1.6m (£1m) in consulting fees and demanding release of the contracts.
Mr Gingrich's contract with the Federal Home Loan Mortgage Corp specifies that the former House Speaker would be paid a monthly retainer of $25,000, for a total of $300,000 for "consulting and related services".
Mr Romney also suggested that Mr Gingrich worked for companies that potentially benefited from the passage of a prescription drug benefit in 2003.
At the time Mr Gingrich was no longer a member of the House of Representatives and did not register as a federal lobbyist, however, he continued to be active on Capitol Hill as he ran a variety of foundations and private groups.
Mr Romney released a television advertisement with similar themes in Florida on Friday - his first official attack ad.
"It's not true," Mr Gingrich said of Mr Romney's assertions. "He knows it's not true. He's deliberately saying things he knows are false."
Mr Gingrich himself spent the days before the South Carolina primary calling on the former Massachusetts governor to release his tax returns - something Mr Romney has now said he will do on Tuesday.
Mr Gingrich won a decisive victory in South Carolina on Saturday, taking 40% of the vote, and the Gingrich campaign said it received $1m in 24 hours after his victory in South Carolina.
The Gingrich campaign's new infusion of cash will help in a state where television ads are crucial, but his campaign is still under-funded compared to Mr Romney.
Texas Congressman Ron Paul will skip the state, focusing on other upcoming primaries.
Mr Santorum, however, will campaign in Florida in an attempt to stay in the race after a third-place finish in South Carolina.
The Florida primary will be held on 31 January.
With Newt Gingrich's win in South Carolina, the nomination contest is now expected to continue into the spring.
The eventual Republican candidate will face President Barack Obama in the general election in November.