JACKSONVILLE, Fla. -- In an energized debate that was as much about projecting a powerful and presidential image as it was about answering questions, the four candidates for the Republican nomination Thursday night defended their stated positions and traded jabs in a final pitch for votes in Florida's influential primary Tuesday.

At stake is the claim to momentum -- and 50 precious delegates -- going into the month before Super Tuesday and what could be a drawn-out battle for the opportunity to challenge President Barack Obama in November.

The debate at the University of North Florida pitted the two front-runners -- former House Speaker Newt Gingrich and former Massachusetts Gov. Mitt Romney -- who have been at each other's throats in political advertising that has saturated the Sunshine State.

But it might have been former Pennsylvania Sen. Rick Santorum and Texas Rep. Ron Paul who fared best in the debate by staying clear of the fray while seizing every opportunity to assert themselves. Still, with little ground game in Florida, it is unlikely that either candidate can burst forward in the days that remain before the state's winner-take-all primary on Tuesday.

Mr. Santorum kicked off the debate to cheers when he introduced his 93-year-old mother, a resident of Florida, and Mr. Paul shattered numerous strings of policy wrangling with light comments.

The debate became frisky early when Mr. Gingrich turned the topic of illegal immigration into whether Mr. Romney advocated rounding up grandmothers and throwing them out of the country.

"I am prepared to be very tough and very bold, but I'm also prepared to be realistic," said Mr. Gingrich, who called Mr. Romney the most "anti-immigrant" of the four candidates. "I do believe there needs to be some level of humanity for people who have been here a long time."

"Mr. Speaker, I'm not anti-immigrant," Mr. Romney shot back. "My father was born in Mexico. My wife's father was born in Wales. They came to this country. The idea that I'm anti-immigrant is repulsive. Don't use a term like that.