Polling booths have opened in Florida, the largest state so far to take part in the Republican race for the White House, with Mitt Romney holding a solid lead over Newt Gingrich in the latest polls.
Voting precincts opened at 7am (12:00GMT) on Tuesday, in the winner-take-all Sunshine State, which offers the prize of 50 delegates - the biggest bounty yet in the contest for the chance to challenge Barack Obama in November.
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On the eve of the vote, former Massachusetts governor Romney led former House speaker Gingrich by 13 points, according to polling averages by non-partisan media company Real Clear Politics, although according to one local poll, the gap may be as much as 20 points.
However, Al Jazeera's John Hendren, reporting from Hollywood in Florida, said it was still to early to say where the votes were going.
"If you speak to voters on the issue of the economy, that seems to be working well for Romney," Hendren said.
"Florida is important because of size, because a big chunk of delegates from this state. Romney faced a big challenge with Gingrich, but according to many polls, his [Gingrich's] popularity seems to have flared and then dipped."
A Romney victory could deliver a crushing blow to Gingrich's campaign following his victory in the South Carolina primary on January 21.
The voting comes after a week of brutal campaigning that saw Romney and Gingrich exchange a barrage of negative and increasingly personal attacks.
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Romney this week attacked Gingrich for his ties to the mortgage agency Freddie Mac and the collapse of the housing market - an open wound in Florida, where residential property values have plunged about 45 per cent since early 2006.
Gingrich picked up the endorsement on Saturday night of former rival Herman Cain, who dropped out of the Republican race in December after allegations of sexual harassment and an extramarital affair.
But polls have shown the race moving toward Romney in Florida for days, reversing the momentum that Gingrich built after scoring an upset by defeating Romney in South Carolina.
Republican presidential candidate Rick Santorum said Gingrich shouldn not be urging him to drop out of the primaries.
The former Pennsylvania senator was responding to remarks by Gingrich suggesting other conservatives need to coalesce around him to keep Romney from winning the party nomination.
Santorum said he was the better, more conservative candidate himself, but that would not justify him asking Gingrich to quit. "Everybody should run," said Santorum.
Santorum, who won a close victory in the Iowa caucuses, lags in the polls and has given up on Tuesday's Florida primary, moving on to other states including Colorado and Nevada.