Republican U.S. presidential candidate Mitt Romney on Monday sharpened his attacks on rival Newt Gingrich's business past and character ahead of a crucial debate in which Romney needs a strong performance to recharge his bid for the White House.

Romney blasted Gingrich on Monday as an erratic politician who has switched positions almost like a pinball machine, in a toughening of his rhetoric to try to halt his chief rival's surprising momentum.

Seeking to regain his footing after losing Saturday's South Carolina primary badly to Gingrich, Romney challenged the former speaker of the U.S. House of Representatives to return the $1.6 million (1 million pounds) in consulting fees he made from Freddie Mac and detail the work he performed for the troubled mortgage giant.

It looked to be a tit-for-tat disclosure tactic on Romney's part after Gingrich effectively attacked him last week for not releasing his tax returns. Romney, who is worth some $270 million, will disclose two years of returns on Tuesday.

New opinion polls show that Gingrich has now jumped into the lead in Florida ahead of the state's pivotal January 31 primary, the fourth contest in the state-by-state battle for the Republican nomination to face President Barack Obama, a Democrat, in the November 6 election.

A Rasmussen Reports telephone survey of likely Republican voters put Gingrich on top of Romney by 9 percentage points and an Insider Advantage poll showed Gingrich with a lead of 8 percentage points.

Romney, the former Massachusetts governor who also previously led private equity firm Bain Capital, hit Gingrich hard on character issues. He warned Republicans that there might be enough baggage in Gingrich's past that could hurt the party's chances of taking back the White House from Obama.

Romney's campaign released a TV ad to criticize Gingrich's Freddie Mac ties and link them to Florida.

'FAMILIES LOST EVERYTHING'

While Florida families lost everything in the housing crisis, Newt Gingrich cashed in, the ad said.

Just as Romney was on the defensive over taxes last week, Gingrich had to explain his Freddie Mac role on Monday.

In an interview on ABC's Good Morning America, Gingrich said Romney was not being honest when he called him a lobbyist for Freddie Mac. Gingrich also said he would work to get his consulting contract with Freddie Mac released.

I did no lobbying, period, Gingrich said. He keeps using the word 'lobbyist' because I'm sure his consultants tell him it scores well. ... He knows it's not true. He's deliberately saying things he knows are false.

Gingrich predicted the next week leading up to Florida's will be combative. He was proven right by Romney on Monday.

Romney has gone into attack mode to try to stop the momentum Gingrich has built up after crushing Romney in South Carolina's primary. Romney has now won just one of the first three states in the nominating process.

The loss punctured the narrative that Romney was in position to win the Republican presidential nomination after easily capturing New Hampshire and coming in a close second in Iowa. It has left Romney scrambling to win Florida - a state where he had once enjoyed double-digit leads in polls.

Romney's use of erratic to describe Gingrich, on the day he will debate Gingrich and other Republican candidates, recalled similar language used by Obama's presidential campaign in 2008 to raise questions about Senator John McCain's age.

As an example, Romney said Gingrich had voted in favour of establishing the U.S. Education Department but said in a debate that we should get rid of the department of education. Gingrich has criticized the Massachusetts healthcare plan Romney developed as governor, but Romney said he had written a couple of years ago about what a superb system it was.

'HIGHLY ERRATIC'

I think as you look at the speaker's record over time, it's been highly erratic, Romney told reporters. He's gone from pillar to post almost like a pinball machine, from item to item in a way which is highly erratic.

It does not suggest a stable, thoughtful course which is normally associated with leadership, he said.

Romney's South Carolina loss has forced him to sharply change strategy from a near all-out focus on Obama as he looked ahead to the general election. Instead, he now must take Gingrich's challenge seriously.

Romney, who was put on the defensive in South Carolina over whether he would release his personal tax documents, plans to release them on Tuesday and is using that development as a point on which to land a blow on Gingrich.

His strategy is to draw attention to Gingrich's years as a Washington insider when he made a fortune from relationships developed when he was House speaker.

He said Gingrich should detail all information he has about his contacts with Freddie Mac as well as the circumstances of an ethics investigation when Gingrich was House speaker.

It is important to get out potentially damaging information now so it does not come up in a general election campaign against Obama, a so-called October surprise, said Romney.

We could see an October surprise a day from Newt Gingrich, he said.

Meanwhile, Gingrich kept up his claim that he - not Romney - is the one who could help change the country.

If people want somebody who's going to shake up Washington, I think I can do it, he said on ABC. If they want somebody who's going to be timid and manage the decay, they ought to vote for Romney.

(Additional reporting by Susan Heavey; Writing by Steve Holland and Deborah Charles; Editing by Mary Milliken and Will Dunham)