By John Whitesides

WASHINGTON (Reuters) - Mitt Romney is back on track.

Less than a week after a stinging setback in South Carolina, Romney moved ahead of rival Newt Gingrich again in Florida polls on Thursday and turned in his strongest debate performance yet in a seesawing Republican presidential race.

Three new polls showed Romney taking a solid 7- or 8-point lead in Florida hours before his confident and aggressive debate performance put Gingrich on the defensive repeatedly in their final showdown ahead of Tuesday's state primary.

"This was his best debate exactly when he needed it. Romney won the debate and he may well have won the primary," Republican strategist Ron Bonjean said.

Romney, who saw his lead over Gingrich evaporate after a stinging 12-point defeat in South Carolina, also benefited on Thursday from a wave of new criticism of Gingrich from prominent conservative and party leaders.

It was the latest swing in momentum in a race that has seen many ebbs and flows. Gingrich earlier wiped out Romney's lead in Florida polls after his South Carolina win, and the potential remains for more shifts in momentum.

But a Florida victory for Romney would put him in a strong position to capture the nomination, with the primary map tilting in his favor in February with contests in seven states where he has the potential advantage.

Next up on February 4 is Nevada, which has a big Mormon population and where Romney, who is Mormon, won with 51 percent of the vote during his failed 2008 presidential bid. On February 7 Minnesota, Colorado and Missouri hold contests. Gingrich did not make the ballot in Missouri.

Four states with February contests - Nevada, Maine, Colorado and Minnesota - use caucus systems, where strong campaign organizations can help rally voter turnout. That could give Romney, with his superior financial and staff resources, an advantage.

On February 28, Michigan and Arizona hold primaries. Romney, the former Massachusetts governor, was raised in Michigan, where his father was a former governor and car executive.

"If Mitt Romney wins in Florida, he'll be on cruise control all the way to Super Tuesday," Republican strategist Ford O'Connell said. Nine states hold contests on "Super Tuesday," on March 6.

"Florida is a make-or-break state for Gingrich because he needs to win to get his momentum back and restock his campaign coffers," O'Connell said. "He is spending everything he's got to compete with Romney in Florida."

'ROMNEY ATTACK MACHINE'

Gingrich, the former House speaker, blamed the "Romney attack machine" for a blizzard of warnings that a Gingrich win in Florida could put him on the path to a nomination that would doom the party against President Barack Obama in November.

"It is now time to take a stand before it is too late," Robert Dole, a former Senate leader and 1996 Republican presidential nominee, said in a statement.

"If Gingrich is the nominee it will have an adverse impact on Republican candidates running for county, state, and federal offices," he said. "He was a one-man-band who rarely took advice. It was his way or the highway," Dole said, referring to Gingrich's tenure as House speaker in the 1990s.

The bombastic conservative columnist Ann Coulter also warned a Gingrich nomination could mean a second Obama term in the White House. "Conservatism is an electable quality," Coulter wrote. "Hotheaded arrogance is neither conservative nor attractive to voters."

The one-two punch of conservative criticism and a strong debate could be a crippling political blow for Gingrich.

"Romney's outstanding debate performance combined with the right-wing establishment coming out against Gingrich is a recipe for disaster for him," Bonjean said. "That was a very effective one-two punch."

Sarah Palin, the 2008 Republican vice presidential nominee, said Gingrich was being vilified by the party's establishment.

"They're trying to crucify this man and rewrite history, and rewrite what it is that he has stood for all these years," Palin told Fox Business Network.

Romney's restored confidence was evident early in a heated exchange over how to handle the estimated 11 million illegal immigrants. Gingrich called Romney the most anti-immigrant of the remaining candidates, which Romney labeled "inexcusable, inflammatory and inappropriate."

When Gingrich insisted Romney's policies would lead to rounding up and deporting grandmothers who had built lives in the United States, Romney shot back: "You know, our problem is not 11 million grandmothers."

(Editing by Alistair Bell and Eric Beech)