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Mitt Romney is now the Republican to beat, after a decisive win in the Florida primary that steals the momentum from his leading rival Newt Gingrich.

Romney took nearly half the vote in Florida Tuesday, winning all 50 delegates the state will send to the Republican National Convention in August.

According to unofficial results released overnight, Gingrich trailed with 32 per cent of the vote, beating the 13 per cent who voted for Rick Santorum and 7 per cent for Ron Paul.

In his victory speech, Romney took aim at his would-be rival in the White House.

"Primary contests are not easy and they're not supposed to be," Romney told supporters in Tampa.

"A competitive primary does not divide us, it prepares us, and we will win .... Mr. President, you were elected to lead, you chose to follow, and now it's time for you to get out of the way."

The run-up to the Florida vote was nothing, if not "competitive."

Reporting from Tampa, NBC News correspondent Tracie Potts said the gloves were off as Romney poured $15 million into ads bashing Gingrich, and Newt fired back with his approximately $3-million campaign.

According to the Kantar Media Campaign Analysis Group, negative ads accounted for 92 per cent of all campaign commercials in the past week. Sixty-eight per cent were aimed at Gingrich.

"As much as these candidates talk about not campaigning negatively, it got pretty down and dirty here in Florida, especially with the ads -- not just from the campaigns, but from the super PACs which are supporting them," Potts told CTV's Canada AM.

Those so-called 'super' political action committees, created in the wake of a 2010 U.S. Supreme Court ruling that removed restrictions on corporate and union spending in federal elections, are attracting donations far in excess of the limited donations that can be made directly to candidates.

While they're forbidden from co-ordinating with the candidates they support, many of the super PACs are staffed with their former campaign workers.

Until Romney's win in Florida, however, it was unclear whether the PACs' spending was paying off.

So far, this is the first time three different candidates -- Santorum in Iowa, Romney in New Hampshire and Gingrich in South Carolina -- have won the first three contests of the Republican nomination race.

With two wins now in his pocket, Romney will be riding that momentum to the next contest in Nevada.

Santorum was already there when he made his concession speech Tuesday night.

"Republicans can do better," he said. "What we saw in the last few weeks in Florida is not going to help us win this election."

Ron Paul is also confident about his chances in the Nevada caucuses, having finished second there during his 2008 campaign for the presidential ticket.

But Romney, with the support of much of the Republican party, is typically seen as the strongest contender to face off against Obama in the next election.

He has had trouble gaining traction with conservative Republicans, however, who question where he stands on key issues such as abortion, gay rights and gun control.

The health insurance plan in Romney's home state of Massachusetts also bears a close resemblance to Obama's national plan, which is widely despised by Republicans.

Romney also faced major questions about his vast personal wealth earlier in the campaign, but eventually released his tax records -- a move that largely shut down the criticism.

Romney has characterized his opponent as an untrustworthy influence peddler with ties to mortgage giant Freddie Mac.

Gingrich has painted Romney as an imposter and has vowed to fight to the end, regardless of the attacks he faces.

"He can bury me for a very short amount of time with four or five or six times as much money," Gingrich said in a television interview. "In the long run, the Republican Party is not going to nominate ... a liberal Republican."