Mitt Romney unleashed his harshest personal attack of the campaign on Newt Gingrich here Sunday, saying “it was proven that he was a failed leader” as the former governor pivoted sharply to regain his footing in the wake of Gingrich’s stunning resurgence over the weekend.

As the dramatically altered presidential campaign moved to Florida, Republican leaders braced for a long and potentially bitter nominating contest. After upending the campaign in South Carolina with a commanding victory, Gingrich moved quickly to capitalize on it. He launched an aggressive fundraising blitz — aides said his “money bomb” raised $1 million in eight hours — and hired new staff to help him compete with Romney as the contest moves to a much more expensive front.

Romney, meanwhile, tried to recast the race as a choice between two kinds of leaders. “We’re not choosing a talk show host,” Romney told an evening rally of more than 500 in Ormond Beach. “We’re choosing the person who should be the leader of the free world.”

“Speaker Gingrich has also been a leader,” Romney said. Then he brought up the ethics investigation into the former House speaker. “At the end of four years, it was proven that he was a failed leader, and he had to resign in disgrace. I don’t know whether you knew that. He actually resigned after four years in disgrace.”

Romney also sought correct his handling of his taxes, saying he would release his most recent returns on Tuesday and acknowledging his previous reluctance to do so was a “mistake.”

Heading to Florida, the two front-runners drew their battle lines for a debate Monday night, when Romney will be under intense pressure to blunt Gingrich’s momentum and do what has long eluded him: capture the enthusiasm of the Republican base.

Gingrich did so last week in South Carolina, but the challenge for the former House speaker is to stir the same passion in the far more populous and politically diverse state of Florida, where expensive media markets require deep pockets and where Romney’s well-financed campaign has been blanketing the airwaves for weeks.

Some Republican leaders were increasingly worried about the growing possibility that the combative Gingrich could lead the GOP into the fall general election — or further divide the party trying to get there. Gingrich sought to turn those concerns to his advantage by stoking the anti-establishment currents running through the Republican electorate.

“We are going to make the establishment very uncomfortable. We are going to demand real change in Washington,” Gingrich said on NBC’s “Meet the Press.” “People are just sick and tired of being told what they’re allowed to think, what they’re allowed to say. As they look at the big boys in Wall Street, as they look at Washington, they know none of the help got down to average everyday Floridians, and I think that gap creates a real anger against the national establishment.”

Romney is trying to tie Gingrich to Washington; his chief strategist Stuart Stevens referred to him repeatedly as a “congressman from the ’90s.” Yet Romney faces his own hurdle in doing so: some Republican voters see Romney as an inside-the-Beltway figure, even if he has never lived there, because of his establishment credentials and endorsements by party elders.

“I think Romney is the clear inside-the-Beltway candidate,” said former Nevada governor Bob List, who is unaligned. “The Washington establishment seems to be the core opposition to Gingrich. I don’t hear people out here criticizing him — ‘Oh, he’s volatile’ and ‘Oh, he’s unstable’ — like I do from the Washington crowd.”

Even as Republican leaders saw a two-man race taking shape, former senator Rick Santorum (Pa.) arrived in Florida after a distant third-place finish in South Carolina and lashed out at both Romney and Gingrich. Santorum said the race was only just beginning and predicted he could win as “a strong conservative — the kind that’s not going to blow up or blow away.”

“The inevitability of Romney has now been wiped away,” Santorum said following a rally in Coral Springs. As for Gingrich, he said the former speaker was an unreliable conservative. “He didn’t live up to all the hype,” Santorum said. “It’s great to be glib. It’s better to be principled.”

Rep. Ron Paul (Tex.), who will share the stage at Monday’s debate in Tampa, is bypassing the Florida primary to focus on a series of smaller February caucuses, starting in Nevada, where he has a loyal and organized following.

Gingrich’s priority, meanwhile, is converting Saturday’s momentum into financial and organizational muscle. “They’re watching the little tick-tick-tick meters on their iPhone applications,” Gingrich spokesman R.C. Hammond said, referencing Gingrich’s “money bomb” fundraising campaign.

On the stump this week, Gingrich intends to talk about the federal space program, Social Security, U.S. policy toward Israel and the economy, Hammond said.

In a state with a sizable Hispanic population, Gingrich and Romney both plan extensive Latino outreach. Gingrich’s campaign has hired two former senior advisers to Sen. Marco Rubio and several other Hispanic leaders. Romney has the endorsements of three prominent Cuban-American congressmen and earlier this month began running a Spanish-language television advertisement narrated by his Spanish-speaking son, Craig.

For weeks, Romney’s campaign has been targeting the more than 450,000 Floridians who requested absentee ballots with plans to individually contact each of them at least twice. And Romney is staffing up in the states holding contests in February and March, girding for a long race to amass delegates.

“We always knew it would be a long process and the Romney campaign seems to be the only campaign prepared for that,” said former senator Norm Coleman (Minn.), a Romney supporter. “I think Mitt is going to be a stronger candidate, more battle-tested, and the party will come together in the end.”

Gingrich’s advisers tried to play down expectations in Florida, highlighting the sheer amount of money and organizing strength Romney has in the state. “It’s always good to be a little bit underestimated, and we have some strengths there that perhaps are not yet apparent,” Gingrich adviser Kevin Kellems said.

So-called “super PACs” supporting both front-runners plan aggressive television advertising campaigns. The Gingrich super PAC is expected to once again attack Romney’s work for venture capital firm Bain Capital, this time focusing on its takeover of Damon Clinical Laboratories, a company that was fined nearly $120 million amid accusations of Medicare fraud.

Romney said the South Carolina campaign “was not a great week for me” and acknowledged that the focus on his wavering comments about his tax returns contributed to “a setback.” He said he would release his 2010 returns and a 2011 estimate online Tuesday.

“We made a mistake in holding off as long as we did,” Romney said on “Fox News Sunday.”

Later Sunday, Romney’s wife, Ann, stepped up ahead of the release of his tax returns to tell Floridians “where we know our riches are.”

“Our riches, you can value them in the children we have and in the grandchildren we have,” she said. “That’s where our values are, that’s where our heart is and that’s where we measure our wealth.”

Although Romney’s surrogates have questioned the thrice-married Gingrich’s character, the candidate himself has rarely brought it up. In the Sunday interview and rally here, Romney drew a sharp contrast with Gingrich.

“I believe leaders have integrity,” Romney said at the rally here. “I believe they’re people of sobriety, judgment, thoughtfulness, reliability, high ethical standards — all the elements of leadership, I think you’re going to have to look at as Floridians.”

Romney said Gingrich has spent the last 15 years since resigning as speaker “selling influence around Washington.” He called on Gingrich to release his records as a consultant to Freddie Mac and tried to blame him for the housing crisis.

“What was his work product there?” Romney said. “What was he doing for Freddie Mac? Because Freddie Mac figures in very prominently to the fact that people in Florida have seen home values go down.”

Romney’s advisers said he intends to confront Gingrich aggressively at the Monday debate and will lay out their policy differences in an economic-themed speech Tuesday.

“It’s a very, very different race moving onto Florida now,” said Stevens, the Romney strategist. “It’s a contrast between a governor with executive experience, a businessperson, versus solutions from Washington. It takes a while sometimes for that to come into focus, but that focus isn’t going to go away.”

Rep. Jason Chaffetz (R-Utah), a Romney supporter who has been campaigning with him, said Gingrich’s two failed marriages and recent interviews with his second wife, Marianne, in which she alleged that Gingrich asked for an “open marriage,” would hurt Gingrich in the Florida contest.

“If we put up anything less than a Boy Scout, we’re gonna be in trouble,” Chaffetz said in an interview. “I don’t believe that voters in Florida will find that media bias trumps serial philandering.”

Gingrich angrily denied his ex-wife’s claim at last Thursday’s debate in an electric exchange with the moderator that was seen as helping him win over undecided voters. Among the biggest surprises of Gingrich’s South Carolina win was his wide support among women, suggesting he may have dispensed with a topic long thought to be a weakness for him — his history of extramarital affairs.

Gardner reported from Washington. Staff writer Rosalind Helderman contributed from Coral Springs, Fl., and staff writer Sandhya Somashekhar contributed from Washington.