After a bruising clash in South Carolina, Republican presidential frontrunners Mitt Romney and Newt Gingrich will take their battle to a bigger stage when the campaign moves to Florida on Sunday.

Gingrich, a former U.S. House of Representatives speaker, thrashed Romney in the South Carolina primary on Saturday, suggesting the race for their party's nomination and the right to face President Barack Obama in November may last months more.

The largest of the early voting states by far, Florida presents logistical and financial challenges that appear to give an advantage to Romney's well funded campaign machine.

But Gingrich has momentum after coming from behind in South Carolina to win around 40 percent of the vote, followed by Romney with 28 percent. Rick Santorum, a former U.S. senator, was in third with 17 percent and U.S. congressman Ron Paul in fourth with 13 percent.

We proved here in South Carolina that people ... with the right ideas beats big money, Gingrich told supporters after his victory in the conservative state.

After strong performances in a series of debates, Gingrich was seen by South Carolina voters as the most likely Republican to beat Obama, a Democrat, in the November 6 election.

They also rejected millionaire former businessman Romney's pitch that he is the best bet to fix a broken U.S. economy and win the White House.

Romney and Gingrich, who have attacked each other mercilessly in a series of negative television ads since December, face off in a debate in Tampa, Florida, on Monday night.

ROMNEY TAX SOLUTION?

Romney has stumbled over questions about his personal finances in recent debates and acknowledged last week that he only pays a 15 percent tax rate, much lower than that of most working Americans.

The former Massachusetts governor has so far resisted calls from rivals, and even ally New Jersey Governor Chris Christie, to release his tax returns.

To try to put the tax return controversy behind him, the Romney campaign has a plan to settle the issue next week, a Republican official said.

That is part of a strategy to be more aggressive against Gingrich, a formidable debater who nevertheless has personal and professional baggage that the Romney team could exploit. Romney accuses Gingrich of being a Washington insider.

The choice within our party has also come into stark focus. President Obama has no experience running a business and no experience running a state. Our party can't be lead to victory by someone who also has never run a business and never run a state, Romney said on Saturday.

Romney saw his aura of inevitability erode in South Carolina after leading opinion polls by 10 percentage points a week ago.

In Florida, he leads Gingrich by 40.5 percent to 22 percent, according to a poll of polls by RealClearPolitics.com. Santorum, a social conservative who is from Pennsylvania, is third with 15 percent.

Campaigns must spend at least $1 million each week to reach voters in the sprawling southern state, according to local political officials. Romney's allies have already spent $5 million, mostly on ads attacking Gingrich. No other candidate has a significant presence in the state.

(Editing by Paul Simao)