TAMPA, Fla. — Mitt Romney, facing a restive Republican Party and a resurgent Newt Gingrich, shifted course and agreed to release his tax returns this week, as the two candidates and their allies buckled in Monday for a combative and unpredictable new phase of the presidential nominating campaign.
As they prepared for a debate here Monday night, the two campaigns were trading charges over the Congressional ethics inquiry on Mr. Gingrich in the 1990s, over Mr. Romney’s tax returns and over whether Mr. Gingrich’s consulting work for the government-sponsored mortgage lender Freddie Mac amounted to lobbying.
A day after telling voters in Florida, “We’re not choosing a talk show host,” Mr. Romney sought to link his rival to the state’s soaring foreclosure rate. He sat down with eight people here on Monday morning for a conversation about the depressed housing market, a situation he called “just tragic.”
“It will get better,” Mr. Romney said. “We’ll not always be like this. This is a detour for America’s history.”
With Mr. Gingrich heading to Tampa for an afternoon rally, hoping to capitalize on the momentum from his commanding weekend victory in the South Carolina primary, the next battleground is the Florida primary on Jan. 31.
But their intensifying duel over the next eight days is also shaping up as a proxy battle in the fight between the Republican Party’s establishment wing — in favor of Mr. Romney — and a grass-roots insurgency that, for now at least, seems to be coalescing around Mr. Gingrich as a no-holds-barred opponent to President Obama in the fall.
“He’s done something that most people don’t really understand,” said Mallory Factor, a Republican activist based in South Carolina who organizes a weekly meeting of conservative donors in New York City. “He has channeled the frustration of the Republican base and independents.”
Mr. Gingrich moved quickly to take advantage of his victory with a new fund-raising push that he told supporters in a Twitter message had already brought in more than $1 million by late afternoon Sunday.
But perhaps the biggest question hanging over Mr. Gingrich’s ability to prevail in Florida is whether Sheldon Adelson, the billionaire casino owner who bankrolled the “super PAC” Winning Our Future that ran negative ads against Mr. Romney in South Carolina, will write another multimillion-dollar check to finance similar attacks in Florida, where airtime is expensive.
Mr. Romney’s decision to release his tax returns on Tuesday — after weeks in which he had equivocated about whether and when he would release them — reflected the loss of what had seemed to be his commanding front-runner’s position. After saying just days ago that he would release them in April, Mr. Romney said on “Fox News Sunday” that on Tuesday he would make public his return from 2010 and an estimate of the numbers in his 2011 return.
Moving to strip Mr. Gingrich of his anti-establishment credentials, Mr. Romney and his campaign on Sunday began a more assertive effort to define Mr. Gingrich as a Washington insider whose tenure as House speaker was marked by chaos and ethical lapses.
They pressed for more information about the ethics investigation into Mr. Gingrich and portrayed him as part of a special interest culture. Speaking of Mr. Gingrich at an outdoor rally in Ormond Beach, Fla., Mr. Romney said, “He’s been working as a lobbyist, selling influence around Washington.” He also called him a “failed leader.”
Mr. Gingrich dismissed the claim that he had been a lobbyist. But he belittled Mr. Romney as inauthentic and vulnerable because of shifts away from more liberal positions he once held.
“Governor Romney’s core problem was that as the governor of Massachusetts, he was moderate, which by the standards of Republican primary voters is a liberal. And he can’t relax and be candid,” Mr. Gingrich said on “Face the Nation.” “He’s been dancing on eggs trying to figure out how to find a version of Romney that will work.”
As the campaigns made their way to Florida on Sunday, they were looking at a reset landscape where Mr. Gingrich has momentum but Mr. Romney has important financial and organizational advantages.
Mr. Romney and the super PAC supporting him, Restore Our Future, have run millions of dollars in advertisements in Florida for weeks, while Mr. Gingrich and Winning Our Future have run none.
Rick Tyler, a senior adviser to Winning Our Future, said the group was seeking to spend $10 million in Florida; when asked if it had raised enough to meet that goal, Mr. Tyler said, “Not yet.”
It was unclear whether Mr. Adelson would send millions of dollars more to the group.
After Mr. Gingrich came in fourth in Iowa, where he was pummeled by ads by the super PAC supporting Mr. Romney, Mr. Adelson’s $5 million donation to Winning Our Future was central to the revival of his candidacy in South Carolina, aides to him and Mr. Romney agreed.
But it was not certain what steps Mr. Gingrich was taking toward building his organization to meet the prowess of Mr. Romney’s.
Mr. Romney’s campaign aides were preparing to press Mr. Gingrich particularly hard on his consultant work for Freddie Mac, which came in the middle of the housing crisis that ravaged Florida’s real estate market.
Speaking at the rally in Ormond Beach late Sunday, Mr. Romney referred to Mr. Gingrich’s work for the mortgage giant, saying, “Freddie Mac figures in very prominently to the fact that people in Florida have seen home values go down. It’s time to turn that around.”
Mr. Romney’s campaign also indicated that it would continue pressing Mr. Gingrich to share what it maintains are hundreds if not thousands of pages of details relating to the ethics committee investigation, when Mr. Gingrich was accused of inappropriately claiming tax-exempt status for a class he taught at Kennesaw State University in Georgia; the panel alleged that the course he ran had partisan, political aims of pushing his economic platform.
Speaking on CNN on Sunday, Mr. Gingrich said the $300,000 he paid after the investigation was actually a reimbursement of the costs. He said all 1,300 pages of the ethics report were online.
The attacks from the Romney camp seemed intended to stoke concern among party leaders about Mr. Gingrich’s rise.
“He has not had a record of successful leadership,” Mr. Romney said of Mr. Gingrich at the Florida rally.
That theme was pressed by other campaign surrogates, including Representative Jason Chaffetz of Utah, who said in an interview, “With all due respect, the speaker is just not disciplined.”
Henry Barbour, a member of the Republican National Committee from Mississippi, who had been supporting Gov. Rick Perry of Texas but disagreed with Mr. Perry’s endorsement of Mr. Gingrich when he left the race, echoed Mr. Chaffetz’s sentiment. “Gingrich is great for cable news,” he said, “but not the Oval Office.”
For all the attacks against Mr. Gingrich, some of Mr. Romney’s supporters were expressing nervousness about the way his campaign seemed to struggle over the last week, particularly over his tax records.
And for all of Mr. Gingrich’s apparent structural disadvantages in Florida, he will land squarely on Monday in his most comfortable home turf, a television debate stage, this time in Tampa.
Most party leaders agree that the debates are where Mr. Gingrich revived his candidacy, though Liz Cheney, the daughter of former Vice President Dick Cheney and a strategist who is unaligned in the race, said more broadly, “He demonstrated the kind of fighting spirit that people out there who are really worried about a second term for Barack Obama want to see.”
Given the volatility of the electorate, Ms. Cheney warned against ruling out former Senator Rick Santorum of Pennsylvania, who finished third in South Carolina. “It would be a mistake to assume it is a two-man race,” she said.
Jeff Zeleny reported from Tampa, and Jim Rutenberg from Charleston, S.C. Susan Saulny contributed reporting from Ormond Beach, Fla., and Nicholas Confessore contributed reporting from New York.