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 pivotal presidential primary election.

Florida is by far the largest state to hold a primary this year and a Romney victory would give him a big boost in the state-by-state battle to decide who will face Democratic 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 January 21 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 his two strong debate performances and a blizzard of television advertisements attacking Gingrich.

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.

Romney's support in Florida is at 43 percent versus Gingrich at 28 percent, according to Reuters/Ipsos online poll data on Monday. Two other contenders who are no longer campaigning in Florida, former Pennsylvania Senator Rick Santorum and Texas congressman Ron Paul, were at 12 percent and 5 percent, respectively.

On the stump on Monday, Romney was breezy and Gingrich combative, reflecting the respective states of their campaigns. Romney cancelled a scheduled Tuesday morning campaign event; Gingrich scheduled four appearances in a final appeal for support on primary day.

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 on Tuesday Republicans needed to shun Romney and unite behind him 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, Gingrich said in Orlando.

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 fiscally conservative Tea Party, and the movement has been backing Gingrich in the state.

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

Major voting blocs in Florida, including Hispanics, who account for 11.1 percent of the state's Republicans, seem to be heavily favoring Romney.

ROMNEY LEADS AMONG OLDER VOTERS

All of the Republican White House hopefuls have worked hard to appear to retirees, in a state where two-thirds of voters are over 65. A Public Policy Polling tracking poll gave Romney a 12-point lead among older voters.

With many conservative Republicans uncertain about whether Romney is too moderate, he has failed so far to electrify the party. Despite his edge in Florida, Romney still narrowly trails Gingrich in national polls.

Some party leaders worry that a third-party candidate could emerge and take votes from Romney. They also fear a continued lukewarm performance could mean no candidate wins enough to take the nomination outright and delegates pledged to minority candidates could make the difference at the convention.

As Gingrich vows to keep fighting until the nominating convention in August, some voters worried 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 Reuters/Ipsos poll showed.

Early reports on Tuesday were that turnout was low. A poll worker in Orlando, in central Florida, said it was dead. Very few voters appeared early at two stations in Miami Beach, and Harry Sawyer, supervisor of elections for Monroe County, predicted a turnout in the Florida Keys of 25- to 30-percent.

The last polls, in Florida's western panhandle, close at 8 p.m. EST (0100 GMT on Wednesday). Polls opened at 7 a.m. EST(1200 GMT) and close at 7 p.m. EST (0000 GMT Wednesday) in most of the state.

(Additional Reporting by Jane Sutton in Miami, Michael Connor in Hollywood, Florida, Barbara Liston in Orlando, Robert Green in Tampa,; and Paul Simao and Patricia Zengerle in Washington; Writing by Ros Krasny and Patricia Zengerle; Editing by Paul Simao and Ross Colvin)