Mitt Romney spent years building a presidential candidacy based on corporate success, a squeaky clean image and an aura of electability that let him focus on President Barack Obama rather than his GOP rivals.

South Carolina Republicans destroyed that strategy in an instant, saying they see Newt Gingrich, not Romney, as the man best positioned to beat Obama. Romney, who cast aside several moderate positions after leaving the Massachusetts governorship, repositioned himself in a more tactical sense Monday, tearing into Florida like a hungry underdog.

No longer leaving his friends to handle the messy work of attacking Gingrich, Romney lit into the former House speaker with a gusto that changed the campaign's tone and arc in one day. Florida's Jan. 31 primary will prove whether the GOP establishment's buttoned-down favorite can turn himself into a pit bull without appearing desperate, phony or unpresidential.

Campaigning in Tampa, Romney called Gingrich a "highly erratic" operative who possibly engaged in "wrongful activity" as a highly paid Washington consultant.

Then Romney opened Monday night's televised debate by saying Gingrich "had to resign in disgrace" in 1998 after four years as speaker, only to spend the ensuing years "working as an influence-peddler in Washington."

Gingrich's shift in tone was nearly as striking as Romney's, only in the opposite direction. After belittling reporters and electrifying studio audiences in two South Carolina debates, the usually combative Gingrich said Monday he wouldn't waste his time refuting Romney's charges point by point.

"This is the worst kind of trivial politics," Gingrich said dismissively. Nonetheless, he spent several minutes explaining why the $1.6 million he received from mortgage backer Freddie Mac was for consulting work, not lobbying.

He added, somewhat curiously, that his consulting firm brought in a "lobbying expert" to tell employees what was legal and what wasn't. The expert is "prepared to testify," Gingrich said.

The live audience was silent.

After his South Carolina thumping, Romney had little choice but to become the aggressor. Gingrich's sudden nice-guy aura may be slightly riskier, because his fire-breathing performances in South Carolina clearly touched resentful voters who feel Washington's "elites" look down on them.

"Gingrich sees that he is increasingly in the driver's seat in the race, and was not challenged about his personal life, so he did not need to go out aggressively," said Republican strategist John Ullyot. "Less is more at this stage, from his perspective."

Romney still holds several advantages, however, starting with his superior campaign treasury. There's little doubt that much of it will go into TV ads and mailers attacking Gingrich.

"I learned something from that last contest in South Carolina," Romney said in the Tampa debate. "I'm not going to sit back and get attacked day in and day out without returning fire."

Romney himself is now leveling the toughest accusations against Gingrich, rather than leaving them chiefly to allies such as former New Hampshire Gov. John Sununu and a well-funded "super PAC." The super PAC's withering ads on Iowa television nearly wrecked Gingrich's campaign three weeks ago.

Gingrich revived himself with two South Carolina debates in which he made journalists as much a target as Romney and Obama. There were no such fireworks Monday in Tampa.

Romney, Gingrich and the other two candidates _ former Sen. Rick Santorum and Rep. Ron Paul of Texas _ will debate again Thursday night in Jacksonville.

Romney, who made millions with a consulting and corporation-restructuring firm, is bracing for reports Tuesday when he releases his most recent tax returns. The Washington Post and The Wall Street Journal reported late Monday that Romney paid an effective tax rate of about 14 percent on $21.7 million in income in 2010, nearly all of it from dividends or interest from investments.

In Monday's debate, Gingrich _ who paid a higher rate on the $3.1 million he made in 2010 _ showed little interest in pursuing the subject.

When Romney said he would have paid zero taxes under Gingrich's plan to eliminate capital gains taxes, Gingrich calmly said that would be fine, provided Romney used his good fortune to create jobs.

Santorum, who finished a distant third in South Carolina, and Paul, who is not campaigning in Florida, were relegated to the sidelines in what now seems to be a two-person race. Santorum noted that the contest has held many surprises, and took a shot at the two frontrunners.

Romney and Gingrich abandoned conservative principles, he said, by supporting elements of "cap and trade" legislation to curb pollution emissions from industrial sites.

"When push came to shove, they were pushed," Santorum said.

He will struggle to be heard in Florida, which dwarfs Iowa and New Hampshire in terms of size, population and cost of campaigning. The pushing and shoving between Gingrich and Romney will dominate Republicans' attention.

If Romney's newly sharpened elbows don't stop Gingrich's momentum, the Republican establishment will face a hard choice. It can start making peace with the former speaker's mercurial ways and anti-elite rhetoric. Or it can heap even more criticisms on him in a contest that must be prompting at least a few smiles in the White House.

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Charles Babington covers politics for The Associated Press.