By Steve Holland
TAMPA, Florida (Reuters) - Mitt Romney opened fire on surging rival Newt Gingrich on Tuesday, calling him a "lifelong politician" who lacks credibility on how the economy works.
Romney launched his strongest attack to date on the former House of Representatives speaker, who has been gaining ground in polls of Republican voters as the main anti-Romney conservative alternative in the race to determine who will run against President Barack Obama's re-election bid in 2012.
The former governor of Massachusetts, who ran private equity and leveraged buyout firm Bain Capital before going into politics, called Gingrich a typical product of Washington.
"I think to get President Obama out of office, you're going to have to bring something to the race that's different than what he brings. He's a lifelong politician. I think you have to have the credibility of understanding how the economy works. And I do. And that's one reason I'm in this race," Romney said in an interview that aired on Fox News on Tuesday.
Gingrich is perhaps Romney's biggest obstacle to the 2012 Republican presidential nomination after other opponents like Rick Perry, Herman Cain and Michele Bachmann have faded.
Romney is facing criticism by Gingrich and Democrats that he has switched positions on key issues for political gain.
Seeking to take advantage of this criticism of Romney, Gingrich told WSC radio while campaigning in South Carolina on Monday: "I wouldn't lie to the American people. I wouldn't switch my positions for political reasons."
The charge that Romney has flip-flopped on major issues dogged him during his 2008 presidential campaign. It is one factor in why Romney is having trouble attracting conservatives who worry he is too moderate for their tastes.
FIRM ON HEALTHCARE
An example they cite is the healthcare plan he developed for Massachusetts that Obama has said was a model for the U.S. overhaul he pushed through Congress in 2010.
Romney told Fox his continued support for the Massachusetts plan is a sign he is not a political shape-shifter.
"I'm standing by what I did in Massachusetts. I'm not trying to dust it aside. I'm absolutely firm that it was the right thing for our state," he said. "I'll defend that and I understand it has political implications. And if it keeps me from winning a primary, so be it."
Romney swept through Florida, whose January 31 primary election could play a conclusive role in determining which Republican wins the party's nomination to face Obama in next November's presidential election.
On a visit to Miami, Romney picked up the endorsement of three Cuban-American leaders, which could bolster his support among the party's conservative and Hispanic voters. They are Representatives Ileana Ros-Lehtinen and Mario Diaz-Balart and former Representative Lincoln Diaz-Balart.
At that event, Romney was pressed by reporters for a response to a high-profile Democratic ad unveiled on Monday that accused him of flip-flopping.
"Bring it on," Romney said. "We're ready for them."
In Tampa, Romney toured the port and accused Obama of stalling trade deals with South Korea, Colombia and Panama.
The deals, finally approved in October, had been proposed by Republican President George W. Bush but were caught up in partisan wrangling for years. Labor groups had fought them over concerns U.S. jobs would be shipped overseas.
"President Obama put those agreements on hold," Romney said. "Because there were people that supported President Obama who wanted those agreements to be either shelved or slowed down. So trade was used as a political feature, as opposed to something that could create jobs."
Obama's re-election campaign was quick to respond.
Spokeswoman Kara Carscaden said Obama negotiated "smart, fair trade agreements" and "will not sign trade agreements just to sign agreements and will only agree to trade deals that increase jobs and exports for Americans."
Romney is paying most of his attention to winning New Hampshire's January 10 primary but has an active campaign in Iowa, whose caucuses start the nominating process on January 3. He is hoping a win in New Hampshire will catapult him to a strong finish in South Carolina on January 21 and a victory in Florida.
(Additional reporting by Jane Sutton in Miami; Editing by John O'Callaghan and Anthony Boadle)