JACKSONVILLE, Fla. - Mitt Romney, facing his greatest challenge of the campaign so far, relentlessly pressed Newt Gingrich on Thursday night in their final debate before the Florida primary, seeking to regain the offensive against an insurgent candidacy that is unexpectedly threatening to upend his once seemingly indomitable front-runner's status. On immigration, on personal finances and, even, on Mr. Gingrich's proposal for lunar colonies, Mr. Romney gave Mr. Gingrich no quarter, adding prime-time voice to his campaign's all-out assault on Mr. Gingrich that is now running morning, noon and night here. In a debate in which Mr. Romney could ill afford another triumphant night for Mr. Gingrich - who has used the debates to regain his competitive footing this year - he sought to keep Mr. Gingrich from reprising his talent for winning over the debate audience, delivering sharp lines that seemed to draw just as much applause and robbing Mr. Gingrich of a key advantage. "Our problem is not 11 million grandmothers," Mr. Romney said at one point in response to Mr. Gingrich's charge that his immigration policy was inhumane. Our problem is 11 million people getting jobs that many Americans, legal immigrants, would like to have. At another point, he rebuked Mr. Gingrich for calling him "anti-immigrant," saying indignantly, "that's simply inexcusable, it's inexcusable." Mr. Gingrich, who swept into Florida on a wave of momentum from his South Carolina victory, often looked less self-assured than in most of his debates. He pushed back against Mr. Romney, but was on the defensive and shied away from repeating the forceful criticisms he has made against Mr. Romney. How about if the four of us agree for the rest of the evening we'll actually talk about issues that relate to governing America? Mr. Gingrich said when the debate moderator, Wolf Blitzer of CNN, asked him if he was satisfied that Mr. Romney was sufficiently transparent in releasing his tax returns. Wouldn't it be nice if people didn't make accusations somewhere else that they weren't willing to defend here? Mr. Romney said, inviting a fight in which he used offense as defense, telling Mr. Gingrich, "I think it's important for people to make sure that we don't castigate individuals who've been successful." For long stretches at a time Representative Ron Paul and former Senator Rick Santorum were left out of the conversation, which Mr. Santorum tried to turn into an advantage. He assumed the role of referee - or scolding parent - and urged his rivals to do away with "playing petty personal politics." "Guys, leave that alone and focus on the issue," Mr. Santorum said, drawing approval from the Republicans seated in the audience. But Mr. Santorum, in an effort to win over conservatives, sought to raise doubts about Mr. Romney and Mr. Gingrich for previously supporting individual health care mandates. He raised his voice as he repeatedly insisted that the health care plan Mr. Romney signed into law as Massachusetts governor is no different than the national plan President Obama signed. "In Massachusetts, everybody is mandated as a condition of breathing to buy health insurance," Mr. Santorum said. And if you don't, you have to pay a fine. Mr. Romney sought to brush back his opponent, saying, "It's not worth getting angry about." As the crowd fell silent, he said that he would move to repeal Mr. Obama's national health care plan. Sharp as his debate performance was, Mr. Romney had his stumbles. When Mr. Blitzer asked him about a radio advertisement his campaign is running against Mr. Gingrich that alleges Mr. Gingrich said Spanish was the "language of the ghetto," Mr. Romney said, "I doubt that's my ad, but we'll take a look and find out." Mr. Blitzer soon after chimed in, "We just double-checked: It was one of your ads; it's running here in Florida, in - on the radio, and at the end you say, "I'm Mitt Romney, and I approved this ad." " Richard A. Oppel Jr. contributed reporting.