Florida's governor and a former state lawmaker are running for U.S. Senate in a Republican primary race increasingly seen as a collision of establishment party leaders and grassroots party activists -- and the establishment candidate's days of playing nice are over.

Gov. Charlie Crist's campaign told Fox News this week that Marco Rubio, the state's former House speaker, won't get a free pass any longer. With Rubio hitting the front-runner hard and gaining traction, the Crist campaign intends to hit back.

Florida is an important swing state as Republicans look ahead to the 2012 presidential election, and some see the soul of the national party at stake in this 2010 Senate battle between Crist and Rubio

Crist recently added two well-regarded campaign veterans: Eric Eikenberg as the new Crist campaign head and Andrea Saul as the campaign's new communications director. The campaign is focusing on engaging Rubio in counterpunches and initiating punches of its own, an aggressive new strategy that some in the local media have described as negative.

But Rubio, 38, isn't backing down.

"I think Charlie Crist is desperate; he has a serious credibility crisis," Rubio told Fox News. "Desperate people do desperate things, but for us it is not a problem.

"This campaign is about the issues. Whether he likes it or not, we are going to win," he said.

On Wednesday, a reporter in Tallahassee asked Crist why he was "taking the gloves off" against Rubio.

"No reason, no reason, (but) people deserve to know the facts," Crist said.

The facts, the Crist campaign says, are that Rubio has weaknesses on two significant conservative fronts: gun rights and illegal immigration. The Crist campaign charges that Rubio did not do enough on either issue during his tenure as head of the Florida legislature.

The Rubio campaign fired back that Crist is misleading the voting public with distortions of their Rubio's record. The campaign suggests Crist, who essentially had been ignoring Rubio, finally acknowledges a formidable threat to what was considered an easy primary win for the once immensely-popular governor.

Florida GOP insiders say Crist's embrace of President Obama's $787 billion federal stimulus package in February increased his vulnerability within the party. Crist even appeared on stage with Obama at a large, pro-stimulus rally in Florida. 

Conservative critics, vehemently opposed to the stimulus, won't let Crist live that down. And some say Crist has compounded his problems by appearing to waffle on his support of the stimulus in recent weeks. 

The Crist campaign has countered by saying the stimulus package has helped a beleaguered state that's been one of the hardest hit in the current recession. But while some other Republican governors, including Louisiana's Bobby Jindal and Alaska's Sarah Palin, opposed the stimulus, Crist has turned many grassroots conservatives -- and anti-Washington Tea Partiers in and out of the state -- against him.

The National Republican Senatorial Committee, which is backing Crist, stands by his record on the national stage as a successful campaigner and fundraiser. And despite Rubio's momentum, Crist still holds a massive campaign war chest, four times the size of Rubio's, and he continues to retain a double-digit lead in most polls with more than nine months to go until the August GOP primary.