Former Massachusetts governor Mitt Romney held a comfortable double-digit lead in polls over struggling rival Newt Gingrich as Florida's Republican voters cast ballots on Tuesday in a high-stakes presidential primary election.

Florida is by far the largest state to hold a primary so far this year and a Romney victory would give him a big boost in the state-by-state battle to decide who will face Democratic U.S. President Barack Obama in the November general election.

Gingrich, a former speaker of the U.S. House of Representatives, was riding high just 10 days ago after an upset win in South Carolina's primary and led in Florida polls as recently as early last week.

But the well-funded and well-organized Romney took back the lead after two strong debate performances and a blizzard of television advertisements attacking Gingrich.

By Monday, Romney's support in Florida was at 43 percent versus Gingrich's 28 percent, according to Reuters/Ipsos online poll data. Former Pennsylvania Senator Rick Santorum and Texas Representative Ron Paul, who are no longer campaigning in Florida, were at 12 percent and 5 percent, respectively.

Many conservative Republicans, however, remain concerned that Romney is too moderate. The former governor has failed to electrify a party that has reluctantly embraced him while continuing to encourage his more conservative rivals.

Despite his edge in Florida, Romney still narrowly trails Gingrich in national polls. Many analysts say he needs a double-digit victory in the Sunshine State to cement his status as front-runner as the national Republican nomination contest continues.

Romney and his allies spent more than four times as much as Gingrich in Florida.

So for me, Florida's big. But New Hampshire's big too. And I'm hoping as I go to Nevada and Minnesota and Missouri and Colorado and Arizona - and the list goes on and on - that I'll be able to get a lot of support in part because of the response here of people in Florida, Romney told reporters in Tampa.

If Romney wins the nomination but does not catch fire with the Republican base, some party leaders worry that a third-party candidate could emerge and take votes away from him, boosting Obama.

They also fear a continued lukewarm performance could mean no candidate will win enough to take the nomination outright and delegates pledged to minority candidates could make the difference at the convention.

RAISING MONEY

The campaigns and allied Super PAC fundraising groups have until the end of Tuesday to report whose money they are spending, and how, in an increasingly expensive campaign. Campaign finance filings to the Federal Election Commission will for the first time officially show who contributed money to the Super PACs and fueled their multimillion-dollar spending sprees.

A spokesman for Gingrich's campaign said on Twitter that he had raised $10 million (6 million pounds) in the fourth quarter of 2011 and $5 million in January.

If Romney wins in Florida, he would be the first candidate this year to capture more than one of the state-by-state nominating contests. Santorum narrowly won Iowa's caucuses on January 3, Romney won New Hampshire's January 10 primary and Gingrich won in South Carolina on January 21.

Gingrich said Republicans needed to unite behind him and shun Romney if they wanted to defeat Obama on November 6.

If they want to beat President Obama, I believe a conservative has a much better chance, he said in Orlando.

Romney's attacks have focused on Gingrich's work for troubled mortgage giant Freddie Mac, an ethics probe and his resignation as speaker in 1999. They have also mocked Gingrich's attempt to ride the coattails of former President Ronald Reagan, a conservative hero.

Gingrich has derided Romney as a Massachusetts moderate who raised taxes and fees as governor, enforced a healthcare mandate, and will not provide a sharp enough contrast to Obama.

Florida is a stronghold for the small-government Tea Party, and the movement has been backing Gingrich in the state.

But other major voting blocs, including Hispanics, who account for 11.1 percent of Florida's Republicans, seem to be heavily favouring Romney. Crucially, in a state where two-thirds of voters are over 65, a Public Policy Polling poll gave Romney a 12-point lead among older voters.

The fight could go on at least until Super Tuesday on March 6, when Gingrich, a Georgia native, could launch a concerted campaign for the several southern states that will be up for grabs.

But a big Romney victory on Tuesday could leave Gingrich scrambling. Double digits could really scare away potential donors who are looking to replenish Gingrich's coffers, Republican strategist Ford O'Connell said.

As Gingrich vows to keep fighting until the nominating convention in August, some voters worry that a nasty, prolonged primary fight would hurt the eventual nominee.

I don't like any of the candidates, to be honest, said Robert White, 51, a power plant operator in Orlando, who said he had voted for Gingrich, but not enthusiastically. I just kinda like what he stands for. I think Romney has a good chance, but he's running a smear campaign right now.

Florida allows early voting at polling stations and by mail. Thirty-five percent of respondents in the Reuters/Ipsos poll said they had already voted by Monday. They favoured Romney by a 22-point margin, the poll showed.

The last polling stations, in Florida's western panhandle, close at 8 p.m. EST (0100 GMT on Wednesday), an hour later than in most of the state. Polls opened at 7 a.m. EST (1200 GMT).

(Additional Reporting by Jane Sutton in Miami Beach, Ben Gruber in Miami, Michael Connor in Hollywood, Barbara Liston in Orlando, Robert Green in Tampa, Irene Klotz in Melbourne and Michael Haskins in Key West, Florida, and Paul Simao and Patricia Zengerle in Washington; writing by Patricia Zengerle; Editing by Paul Simao and Mohammad Zargham)