Mitt Romney trounced main rival Newt Gingrich in Florida's Republican primary on Tuesday, putting himself in a commanding position to win the nomination to take on President Barack Obama in November, US media projections showed.

The margin of victory - 47 percent to 31 percent with more than two-thirds of the roughly two million votes counted - dealt a bitter blow to former House speaker Gingrich, who faces a Herculean task to turn the race around.

Gingrich, 68, shocked the party establishment when he thumped Romney, 64, in South Carolina earlier this month, but his support sank fast in Florida and Romney is now the one with all the momentum.

While the battle for the right to challenge Obama, a Democrat, in November is still in the relatively early stages, Romney's thumping win in Florida demonstrates his ability to carry a major general election battleground state.

At the end of what had been a caustic Florida campaign, the multimillionaire former Massachusetts governor and venture capitalist summed up what the victory in the large, diverse state meant to him.

"How important is Florida? Today it is the most important thing in the world to me," he told reporters in Tampa, while defending his campaign's tactics and taking a few more shots at Gingrich.

"If we are successful here it will be pretty clear that when attacked, you have to respond and you can't let charges go unanswered."

Gingrich "really can't whine about negative campaigning," Romney added.

"His comments most recently are really quite sad and I think painfully revealing about the speaker and what he is willing to say and do to try to get the nomination."

The atmosphere inside the Tampa ballroom where Romney was to give his victory speech was euphoric as supporters watched the results trickle in, punctuating each gain with chants of "Mitt, Mitt, Mitt!"

"I'm pleased, but I'm not surprised - it is really what I expected," Barbara Jaehne, a doctor from the Florida coastal town of Sarasota, told AFP as the growing crowd waved small American flags to the beat of country music.

"I feel like Newt is very polarizing - he is like the guy on the volleyball team who didn't get picked, he is lashing out and people don't like that."

Gingrich acknowledged he faced an uphill battle in Florida, but promised to wage a long-haul battle for the nomination, taking the fight all the way to the party convention in August when the nominee will ultimately be crowned.

"I think ultimately it's going to come down to a conservative versus Massachusetts moderate primary fight across the whole country," he said at a campaign stop in Orlando.

Gingrich has urged fellow conservative Rick Santorum, who won the Iowa caucuses but has struggled since and looked set to finish a distant third in Florida, to drop out of the race.

Santorum, a former Pennsylvania senator, and libertarian-leaning Texas congressman Ron Paul did little campaigning in Florida.

The economy seemed to be on the minds of many voters, including businessman Paul Jackson, who said he voted for Romney.

"I voted for him because I think he has the best chance of beating the president - the current president. I don't agree with everything that he stands for, but I need to see a change that satisfied my selfish needs as a small business owner," he said.

Florida has been badly hit by the 'Great Recession' wrought by the 2008 financial crisis - unemployment is close to 10 percent and the state was at the epicenter of the housing bubble.

Many residents have seen their homes repossessed or have mortgages now worth more than the value of their homes.