(Reuters) - Mitt Romney was desperate for a strong debate performance in Florida's Republican debate Monday night to restore momentum for his campaign, and delivered repeated sharp attacks on rising rival Newt Gingrich.

Romney lashed out at Gingrich at the Florida debate for resigning as speaker of the House of Representatives under an ethical cloud and repeatedly labeled him an influence peddler.

The former Massachusetts governor needs a victory in the state's primary on January 31 to win back his front-runner status in the fight for the nomination to oppose Democratic President Barack Obama in November's general election after Gingrich's convincing victory in South Carolina on Saturday.

Often lukewarm in earlier debates, Romney kept on message by seeking repeatedly to tar Gingrich as a Washington insider who lobbied for clients including the troubled mortgage giant Freddie Mac.

In the 15 years after he left the speakership, the speaker has been working as an influence peddler in Washington, Romney said.

Gingrich shot back that Romney was not telling the truth. He's said at least four things that are false, he said.

Romney took a much tougher tone than he did last week during a debate in South Carolina, when he stumbled in response to questions about his tax records.

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, Romney said.

On Monday, Romney used a question about his tax returns as an opportunity to outline his proposals to change the tax system by cutting capital gains and corporate taxes. He joked that the country needed to get more people back to work so they could begin to pay taxes.

Romney has promised to release his 2010 tax return and his estimated 2011 taxes on Tuesday.

To distract from his own financial questions, Romney called on Gingrich before the debate to disclose his contract for the government-sponsored mortgage financing giant Freddie Mac, from which Gingrich made $1.6 million.

Gingrich's former consulting firm released a contract he signed with the company in 2006, but it shed little light on what Gingrich was hired to do. The document called for a $300,000 retainer and $25,000 in fees monthly for the year.

The remaining candidates, Texas Congressman Ron Paul and former Pennsylvania Senator Rick Santorum sought to make a mark after finishing far behind in South Carolina, but neither had much of an impact as the two leaders traded insults and barbs.

(Additional reporting by Steve Holland in Tampa and Lily Kuo in Washington; Editing by Peter Cooney and Mohammad Zargham)