(REUTERS) -- Florida's Republican voters go to the polls on Tuesday in a high-stakes presidential primary election that could determine the direction of the race.
In what shapes up as a two-man contest, most opinion polls show Mitt Romney, a former Massachusetts governor, holding a double-digit lead over Newt Gingrich, a former U.S. House speaker.
Florida is the largest state to hold a presidential 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 President Barack Obama in November.
Polls open at 7 a.m. and close at 7 p.m. local time. Most of the state is on Eastern Time, except the western Panhandle region, which is on Central Time.
Just 10 days ago Gingrich was riding high after an upset win in South Carolina. Strong debate performances fueled a come-from-behind win in which Gingrich captured all key demographics.
Florida has been a different story, in part reflecting a torrent of attack ads targeting Gingrich, both from Romney's campaign and from an independent super political action committee.
On the stump on Monday, Romney was breezy and Gingrich combative, reflecting the respective states of their campaigns. Romney cancelled his scheduled Tuesday morning event in Tampa; Gingrich will crisscross the Orlando region making four appearances in a final appeal for support.
We were getting just walked on by Speaker Gingrich and really didn't respond very well in South Carolina, Romney said on NBC's Today show on Monday. So we decided, we're going to respond.
Romney's advertising has focused on Gingrich's work for mortgage giant Freddie Mac, and an ethics probe and his resignation as speaker. It has also mocked Gingrich's attempt to ride the coattails of conservative hero Ronald Reagan.
There's nothing like $17.5 million of false ads to make a big difference, Gingrich told CNN on Monday. I have never seen a candidate for president that methodically dishonest.
By the end of the day on Tuesday, the campaigns and allied Super PACs are due 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.
GINGRICH FALLING IN POLLS
Gingrich, who often touts his prospects for successfully debating Obama, seemed surprisingly flat in two closely watched Florida debates last week. Since then, his popularity in daily tracking polls has fallen.
A Reuters/Ipsos poll released late on Monday showed Romney's support at 43 percent versus Gingrich at 28 percent.
Battling hard, the Gingrich tactic has been to deride 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 Democrat Obama.
When voters hear Gingrich in person they often come away impressed, praising his intellect and toughness.
I was a little undecided between Newt and Mitt but I have decided for sure that Newt's going to get my vote. He's forthright. He says what he means and means what he says, said Gene Vandevander of Tampa, attending a Gingrich rally.
A straw poll of conservative Tea Party sympathizers released on Monday gave Gingrich 35 percent support against former U.S. Senator Rick Santorum at 31 percent and Romney at 18 percent.
Both other key voter groups, including Hispanics, seem to be heavily favoring Romney, whose campaign has run an aggressive ground-game for weeks.
More than 600,000 Floridians have already cast ballots during the state's early-voting period and they favored Romney by a wide margin, the Reuters/Ipsos poll showed.
As Gingrich vows to fight on, some voters worry that a nasty, prolonged primary fight will hurt the eventual nominee.
We don't want to attack each other, but to focus on the issues, said voter Jonathan Sanchez of Orlando. We don't want to seem divided.
Florida's delegates are given on a winner-take-all basis. The two other Republicans on the ballot, Santorum and Texas congressman Ron Paul, have moved on to other states.
After Florida's primary on Tuesday, Nevada's Feb. 4 caucuses are the next contest in the process of choosing a Republican nominee.
(Reporting by Steve Holland and Sam Youngman; Writing by Ros Krasny; Editing by Eric Beech)