ORLANDO/TAMPA, Florida (Reuters) - Anticipating victory in Florida's game-changing Republican presidential primary on Tuesday, Mitt Romney looked ahead while his struggling rival Newt Gingrich vowed to press on with his White House quest.
Florida is the largest nominating contest so far this year and, with double-digit leads in statewide polls, Romney seemed headed for 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.
If Romney wins as expected, it would mark a sharp reversal of fortune for Gingrich. The former speaker of the U.S. House of Representatives scored a shock upset victory over the former Massachusetts governor and private equity executive in the last primary, 10 days ago, in South Carolina.
Gingrich jumped ahead in polls in Florida after that contest, and the party seemed headed for a long primary race.
But the well-funded and well-organized Romney wrested back the lead after two strong debate performances and a blizzard of television advertisements attacking Gingrich.
"(Romney's) a businessman. He knows how to make money. If he makes promises, he'll keep them," said Frank Lobue, a retired utility company employee, who voted in St. Petersburg.
Romney and "Restore our Future", an independent fund-raising group, or Super PAC, that backs him have spent $15 million on advertising in Florida, almost all negative, compared with $4 million by Gingrich and his backers.
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.
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 still narrowly trails Gingrich in some national polls and 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.
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 one will win the nomination outright and delegates pledged to minority candidates could make the difference at the convention.
"We have the time and the calendar and the team and the organization to be able to do what I believe it takes to become the nominee. But I am not going to judge before the process is over. It will be over when those delegates have been collected," Romney told reporters in Tampa.
The campaigns and allied Super PAC groups had until the end of Tuesday to report on their donors and spending. Campaign finance filings to the Federal Election Commission will for the first time officially show who contributed to the Super PACs and fueled their multimillion-dollar spending sprees.
A Gingrich spokesman said on Twitter that he had raised $10 million 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.
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 movement, and it 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 favoring 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 easily go on until "Super Tuesday" on March 6, when Gingrich, who is from Georgia, could launch a concerted campaign for the southern states that will be up for grabs. Only some 15 percent of the delegates will be decided by then.
But a big Romney victory on Tuesday could leave even the determined Gingrich scrambling. "Double digits could really scare away potential donors who are looking to replenish Gingrich's coffers," Republican strategist Ford O'Connell said.
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 had already voted. They favored Romney by a 22-point margin.
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 Cynthia Osterman