Mitt Romney has used the latest GOP presidential debate in Florida to come out swinging against rival Newt Gingrich.

An aggressive Romney repeatedly challenged Gingrich, ridiculing the former House speaker's ties to mortgage giant Freddie Mac, his call to build costly projects in key primary states and to colonize the moon.

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

There were also lighter moments. Ron Paul, when questioned about being 76, challenged his rivals to a 25-mile bike race in the Texas heat. And he said he'd like to send politicians to the moon.

The debate was 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 Santorum and Paul far behind.