ABC News' Amy Walter and Michael Falcone report:

JACKSONVILLE, Fla. - On Thursday night Mitt Romney finally looked like a candidate who wants to win this election. On the debate stage Romney was confident and focused. More important, he was aggressive and disciplined, and never allowed his chief rival, Newt Gingrich, to get the upper hand.

The difference between the Romney who showed up tonight vs. the one who was on stage in South Carolina last week was night and day.

Several of Romney's answers were as knife-sharp as we've seen at any debate during the primary season, particularly his laundry list of projects, including the lunar colony that Gingrich has proposed in the first few early nominating states.

"In South Carolina, it was a new interstate highway, and dredging the port in Charleston. In New Hampshire, it was burying a power line coming in from Canada and building a new VHA hospital in New Hampshire so that people don't have to go to Boston," Romney said. "This idea of going state to state and promising what people want to hear, promising billions, hundreds of billions of dollars to make people happy, that's what got us into the trouble we're in now."

"A big idea," Romney concluded, is not always "a good idea."

That's not to say that Romney's performance at the debate hosted by CNN, the Florida Republican Party and the Hispanic Leadership Network wasn't without fault. When asked to own up to an ad his campaign is running on the radio that said Gingrich once called Spanish the "language of the ghetto," Romney said it wasn't his ad. Only problem: it is. The CNN moderator Wolf Blitzer corrected him publicly a few minutes later.

Romney also repeatedly referred to his "trustee" when asked about his investments, particularly those based offshore.

"I have a trustee that manages my investments in a blind trust. That was so that I would avoid any conflicts of interest," Romney said."That trustee indicated last week, when he was asked about this, he said that he wanted to diversify the investments that I had."

But, it was Newt who fell flat tonight.

With the latest polls suggesting that Gingrich's momentum in Florida ebbing, Newt needed a strong performance at the final debate before voters cast their ballots next Tuesday. He didn't have one.

Gingrich's Achilles heel is his lack of discipline and follow-through. The fired up and aggressive Newt we saw on the campaign trail this week was not on the stage tonight. In his place was a passive and hesitant candidate. Even his attempt at turning the audience against the moderator didn't work.

"This is a nonsense question," Gingrich told Blitzer when the moderator asked whether Romney had been transparent enough in releasing his tax returns. "'He lives in a world of Swiss bank and Cayman Island bank accounts,' I didn't say that.  You did," Blitzer said confronting him with his own words. Again, Gingrich wouldn't bite, and it fell to Romney to attack.

"Wouldn't it be nice if people didn't make accusations somewhere else that they weren't willing to defend here?" the former Massachusetts governor said referring to his rival.

Rick Santorum was the only candidate to challenge Romney, going after him aggressively on his Massachusetts health care plan.

"What has happened in Massachusetts is that people are now paying the fine because health insurance is so expensive. And you have a pre-existing condition clause in yours, just like Barack Obama," Santorum said. "So what is happening in Massachusetts, the people that Governor Romney said he wanted to go after, the people that were free-riding, free ridership has gone up five-fold in Massachusetts. Five times the rate it was before."

Even so, any gain Santorum makes in the polls is likely to come at Gingrich's expense.

To win Florida, Gingrich needs to build on the momentum he got from his South Carolina win. His debate performance was a momentum killer.