Mitt Romney marched ahead with a commanding lead in the US Republican race for the White House after trouncing his main rival Newt Gingrich in a bitterly-fought Florida primary yesterday.

The margin of victory—46 percent to 32 percent — dealt a bitter blow to former House speaker Gingrich, who faces a Herculean task to try to catch up with Romney, a multimillionaire businessman and former Massachusetts governor.

Gingrich, 68, shocked the party establishment when he thumped Romney, 64, in South Carolina earlier this month, but his support sank fast in the larger and more diverse state of Florida, and Romney now has all the momentum.

Romney's double-digit win in the Sunshine State demonstrated his strength in a key general election battleground, and he used his victory speech to urge Americans to evict President Barack Obama from the White House in November.

"Mr. President, you were elected to lead. You chose to follow, and now it's time for you to get out of the way," he said, to chants of "Mitt! Mitt! Mitt!" from hundreds of supporters in a packed Tampa ballroom.

"President Obama wants to fundamentally transform America and make it something perhaps we wouldn't recognise. I want to restore to America the values and principles that made us the hope of the Earth," Romney said.

It was a warning shot at Democrats who have been salivating over the increasingly caustic tone of the Republican campaign — marked in Florida by a heavy exchange of increasingly personal attacks between Romney and Gingrich.

"A competitive primary does not divide us, it prepares us and we will win," Romney said.

But in a sign of how bitter the campaign has become, Gingrich broke with tradition by neither congratulating Romney nor calling him after the vote.

The economy seemed to be on the minds of many voters in Florida, including businessman Paul Jackson, who said he voted for Romney because "he has the best chance of beating the president — the current president."

Despite the stinging defeat, Gingrich promised to wage a long-haul battle all the way to the August 27-30 convention, which will formally crown a nominee to go head-to-head against Obama on November 6.

"This will be a two-person race between the conservative leader, Newt Gingrich, and the Massachusetts moderate," he told supporters in Orlando, referring to Romney.