The stakes are high, but Mitt Romney isn’t showing much concern as he caravans across Florida this weekend with a sudden swagger and the newfound looseness of a front-runner who thinks he’s cleared the obstacles in his path.

Romney is coming out of the toughest stretch of his campaign, which was punctuated by a stinging loss to Newt Gingrich in the South Carolina primary. Now, he has opened a lead in the Florida polls before the state’s primary Tuesday and is aggressively trying to dispatch the former House speaker and focus on President Obama.

But Romney’s task become somewhat harder late Saturday when former candidate Herman Cain, a tea-party favorite, endorsed Gingrich. “Speaker Gingrich is a patriot. Speaker Gingrich is not afraid of bold ideas,” Cain said, appearing with him at a Republican fundraiser in West Palm Beach, Fla.

Meanwhile, former Pennsylvania senator Rick Santorum’s campaign announced that he was canceling his Sunday morning appearances in Florida because his 3-year-old daughter, Isabella, seriously ill with a genetic disorder, has been admitted to a Philadelphia hospital.

Gingrich said Saturday he would go “all the way to the convention” and predicted a “wild and woolly” campaign for the next few days. His well-financed allies showed no signs of letting up their televised assault on Romney, while former Alaska governor Sarah Palin rushed to Gingrich’s defense, in a sign he might mobilize the party’s grass-roots conservatives.

The former House speaker, who has drawn large and enthusiastic crowds all week, said he would become the nominee if he wins Florida. And if he doesn’t, he said, he would be back in Tampa for this summer’s Republican National Convention.

“I will go all the way to the convention,” Gingrich said. “I expect to win the nomination. You just had two national polls that show me ahead. Why don’t you ask Governor Romney what he will do if he loses since he is behind in both national polls?”

But nobody has asked Romney that; the candidate has not held a media availability since last Monday. Instead, Romney is campaigning here with fresh energy and a more carefree spirit.

Earlier this week, he fired methodical attacks at Gingrich designed to rattle him and raise questions about his trustworthiness and leadership skills. But the past few days, while Gingrich takes bitter swipe after bitter swipe at him, Romney has hardly attacked his rival at all. When he has, his lines have been more humorous than high-handed.

In Pensacola on Saturday morning, Romney jabbed his sidekick, John McCain (R-Ariz.) about the mischievous tales of the senator’s Navy pilot youth. He won polite laughter after telling an awkward joke about a pickup driver in Wyoming with stinky animals in the back. He confided his nervousness about making “chitchat” with actor Jon Voight while phoning him to seek his endorsement.

And Romney said nothing about Gingrich other than to liken him to Goldilocks.

“We’ve had about 18 debates so far,” Romney told hundreds of supporters who filled the docks and balconies at a Pensacola restaurant Saturday morning. “They’re getting more and more fun as time goes on. This last one, Speaker Gingrich said he didn’t do so well because the audience was so loud. The one before, he said he didn’t do so well because the audience was too quiet. This is like Goldilocks, you know, you’ve got to get it just right.”

Aides say Romney is having more fun here than he has all during the campaign, and they put that confidence on display here Saturday afternoon by turning up Van Halen and driving Romney’s bus up to the dock at a Panama City shipyard to deliver the candidate to his rally in grand fashion.

“I just feel like things are going in our direction,” Romney said at the rope line after the rally. “I think we are going to win here.”

Romney’s candidacy is on the line in Florida; a loss here to Gingrich would cast fresh and serious doubt on his ability to consolidate the party behind his candidacy. But buoyed by his newly aggressive performance in Thursday’s debate — McCain called it a rare “knockout” — Romney is rallying Republicans in this sprawling swing state with talk not of the primary, but of the fall general election.

“When I debate the president, I’m not going to worry about the audience,” Romney told the Pensacola crowd. “I’m going to make sure that we take down Barack Obama and take back the White House.”

Yet as the sun started to fall Saturday afternoon, Romney returned briefly to his tough talk against Gingrich. “I’m speaking to you today as if I’m already the candidate for the Republican party, but I’m not,” he acknowledged. “I’m running against Speaker Gingrich — a very nice fellow, and he’s a historian. But that doesn’t give him the right to rewrite history.”

Romney revisited the tumultuous end of Gingrich’s speakership, noting that he “resigned in disgrace. . . . We have to go back and look at history and say he may be a great guy with great ideas, but he’s not going to be the leader we need.”

Going after ‘Newtorious’

At Boston headquarters, meanwhile, Romney’s team tried to keep its boot on Gingrich’s neck. Aides launched a Twitter campaign to brand Gingrich “Newtorious.” Romney, for weeks, has been running an aggressive early-voting and absentee-ballot campaign here, holds with his allies a more than 2-1 advantage in television advertising spending.

Romney’s campaign released a tough, new ad in Florida titled “History Lesson,” which entirely consists of Tom Brokaw anchoring a vintage “NBC Nightly News” broadcast about Gingrich being reprimanded for ethics violations as speaker.

NBC News quickly asked the Romney campaign to pull the ad off the air. “I am extremely uncomfortable with the extended use of my personal image in this political ad,” Brokaw said in a statement.

Gingrich’s campaign pounced, too, issuing a statement slamming Romney for “another big lie.” Gingrich aides said the ad neglects to point out that the Internal Revenue Service cleared Gingrich of the “substance” of the ethics charges.

Palin took to Facebook to defend Gingrich in a post titled “Cannibals in GOP Establishment Employ Tactics of the Left.”

“We need a fair primary that is not prematurely cut short by the GOP establishment using Alinsky tactics to kneecap Governor Romney’s chief rival,” Palin wrote, referring to Saul Alinsky, the left-wing community organizer Gingrich often quotes.

The endorsement by Cain, another conservative favorite, could help Gingrich in Florida, where the business executive earned a surprising win in a straw poll last summer — and launched his dramatic, if short-lived, rise to the top of the field. Since he left the race, Cain had been coy about whom he would endorse; last week, he said he was planning to endorse “We The People.” Saturday evening, he said backing Gingrich had been “in my heart and mind a long time.”

Although Gingrich needs any boost he can get in Florida, the timing of the endorsement was odd — too late to make the evening television newscasts, and sandwiched before the expected release of a new NBC/Marist poll widely expected to show a widening lead for Romney.

Gingrich said he hopes Cain will co-chair a commission to lead the policy discussion on the economy and taxes.

On the stump, Gingrich tried to keep his focus on Obama and the stark contrast he believes he would provide with the incumbent. He said the GOP nominated a “moderate” in 1996 and 2008 and lost. “If we nominate a moderate, we are in real trouble,” he said. “It’s that simple.”

Later, addressing a tea party crowd in Winter Haven, Gingrich got a standing ovation for saying that stem-cell research amounts to “the use of science to desensitize society over the killing of babies.”

But Gingrich is struggling to keep his message focused on Obama. At each turn, he kept returning to his grievances against Romney. It hasn’t helped that four congressmen supporting Romney have been following Gingrich to his campaign events to spin the press corps.

At a golf course in Port St. Lucie, Gingrich spokesman R.C. Hammond led reporters and TV cameras to provoke a confrontation with Rep. Connie Mack (Fla.), a Romney supporter, over who would make the better president. When Hammond came over, Mack quickly launched into a browbeating about Gingrich’s work for Freddie Mac, the federally backed mortgage giant that paid his firm $1.6 million for consulting work.

“Is he a lobbyist?” Mack asked. “The Florida voters deserve an answer about what kind of influence he’s been peddling. He won’t answer. Instead, he belittles them.”

Santorum’s campaign said Isabella, known as Bella, who has the genetic disorder Trisomy 18, has been admitted to Children’s Hospital in Philadelphia.

However, the campaign’s statement said, Santorum plans to resume campaigning as soon as possible. His campaign said he currently expects to take part in his scheduled 2 p.m. rally in Sarasota and, as of now, has only canceled his plans to attend a Miami church.

Staff writers Karen Tumulty and Rosalind S. Helderman in Florida contributed to this report.