Jacksonville, Fla. ? An aggressive Mitt Romney repeatedly challenged Republican rival Newt Gingrich in a fast-paced campaign debate Thursday night just before a Florida primary that could be pivotal in the GOP presidential race. He rebuffed criticism of his own stance on immigration and ridiculed the former House speaker’s call to build costly projects in key primary states and to colonize the moon.

Days before the primary election in Florida, a state with a large number of immigrants, Romney vehemently denied Gingrich’s accusation that he is more anti-immigrant than any other candidate. And, as charges flew back and forth, Gingrich in return rebutted any suggestion that he wasn’t the man to rein in federal spending.

“You don’t just have to be cheap everywhere. You can actually have priorities to get things done,” Gingrich declared, saying that as speaker of the House he had helped balance the budget while doubling spending on the National Institutes of health.

The debate was the 19th since the race for the Republican nomination began last year, and the second in four days in the run-up to next Tuesday’s Florida primary. Opinion polls make the race a close one — slight advantage Romney — with two other contenders, former Sen. Rick Santorum of Pennsylvania and Texas Rep. Ron Paul far behind.

Gingrich’s upset victory in the South Carolina primary last week upended the race for the nomination to oppose Democratic President Barack Obama in the fall, and Romney in particular can ill-afford a defeat on Tuesday.

While the clashes between Gingrich and Romney dominated the debate, Santorum drew applause from the audience when he called on the two front-runners to stop attacking one another and “focus on the issues.”

“Can we set aside that Newt was a member of Congress … and that Mitt Romney is a wealthy guy?” he said in a tone of exasperation.

There were some moments of levity, including when Paul, 76, was asked whether he would be willing to release his medical records. He said he was, then challenged the other three men on the debate stage to a 25-mile bike race.

He got no takers.

In the days since Romney’s loss in South Carolina, he has tried to seize the initiative, playing the aggressor in the Tampa debate and assailing Gingrich in campaign speeches and a TV commercial.

An outside group formed to support Romney has spent more than his own campaign’s millions on ads, some of them designed to stop Gingrich’s campaign momentum before it is too late to deny him the nomination.

With polls suggesting his South Carolina surge is stalling, Gingrich unleashed a particularly strong attack earlier in the day, much as he lashed out in Iowa when he rose in the polls, only to be knocked back by an onslaught of ads he was unable to counter effectively.

Thursday night’s first clash occurred moments after the debate opened, when Gingrich responded to a question by saying Romney was the most anti-immigrant of all four contenders on stage. “That’s simply inexcusable,” the former Massachusetts governor responded.

“Mr. Speaker, I’m not anti-immigrant, my father was born in Mexico,” Romney declared. “I’m not anti-immigrant.”

At the same time, Romney noted that Gingrich’s campaign had been pressured to stop running a radio ad that called Romney anti-immigrant after Florida Sen. Marco Rubio called on Gingrich to do so.

He called on Gingrich to apologize for the commercial, but got no commitment.

About an hour later, Romney pounced when the topic turned to Gingrich’s proposal for an permanent American colony on the moon — an issue of particular interest to engineers and others who live on Florida’s famed Space Coast.

A career businessman before he became a politician, Romney said: “If I had a business executive come to me and say I want to spend a few hundred billion dollars to put a colony on the moon, I’d say, ‘You’re fired.'”

The audience erupted in cheers.

Gingrich says NASA should offer prize incentives to businesses that develop commercial flight to the moon and beyond. He holds up that idea as an example of bold thinking.

Paul drew laughs when he said the people he’d send to the moon are politicians.