Romney on the offensive against Gingrich

Mitt Romney has more money than his rivals for the Florida primary (AP)

Mitt Romney yesterday released two years of tax filings, after days of being on the defensive over an issue that has threatened to derail his position as the Republican front-runner to challenge Barack Obama for the presidency later this year.

As expected, they showed he is the richest candidate ever to run for presidency and that he has been taking advantage of federal tax laws to pay lower rates of taxes than most Americans - 13.9 per cent in 2010 - because his income comes almost entirely from his investments.

The documents also revealed some recent politically judicious house cleaning. An account he held with a Swiss bank, the Union Bank of Switzerland, was closed in early 2010. He also withdrew investments made in firms that had business in Iran and China.

The wounds from his defeat in South Carolina at the weekend barely scabbed over, Mr Romney is now racing to tilt the contest for the Republican nomination back in his favour by pressing a furious campaign to discredit his main rival, Newt Gingrich, as an "influence peddler" who left his post as the Speaker of the House in disgrace.

The attacks may have stuck in a televised debate in Tampa on Monday night when the normally irrepressible Mr Gingrich showed weariness. "There's a point in this process where it gets unnecessarily personal and nasty, and that's sad," he said.

As Mr Gingrich and Mr Romney played Punch and Judy for long periods of the debate, the other candidates, Ron Paul and Rick Santorum, were left to look on. Mr Gingrich, who will be under intense pressure to repeat his South Carolina victory in the Florida primary next week, almost conceded yesterday that he had been off form by threatening to boycott future debates if the audiences are forced to stay silent - as was the case on Monday, a change that seemed to deprive of him of oxygen.

All eyes now are on the next leg of the GOP contest. Less monolithically conservative than South Carolina and arguably more representative of the country as a whole, with high unemployment and a shattered housing sector, Florida will be a big challenge for Mr Gingrich. He has less ground organisation in the state and less cash on hand than Mr Romney. In a boost, however, a super-PAC that supports Mr Gingrich has received a new $5m infusion from a Las Vegas benefactor.