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By Jason Szep and Tim Gaynor

JACKSONVILLE, Fla., Jan 28 (Reuters) - In a tight battle in Florida, John McCain and Mitt Romney competed to stick each other with the "liberal" tag, a harsh smear among conservative Republicans whose votes could be decisive in Tuesday's voting for presidential contenders.

Republican candidates scrambled in buses and jets across the nation's fourth-largest state on Monday in a final hunt for votes on the eve of their biggest nomination test yet in the most wide open White House race in more than 50 years.

The winner could ride a wave of momentum into "Super Tuesday" on Feb. 5 when 22 states vote in a blitz of primaries that could anoint a Republican front-runner to contest November's election to succeed President George W. Bush.

Romney, a former Massachusetts governor, branded three of McCain's signature pieces of legislation -- on energy, immigration and campaign finance reform -- as "liberal."

He also sought to associate the Arizona senator with a liberal standard-bearer, Sen. John Kerry, the 2004 Democratic presidential nominee.

In West Palm Beach, Romney reminded voters that McCain once said he would entertain the idea of joining Kerry's 2004 campaign as a vice presidential running mate.

"Had someone asked me that question, there would not have been a nanosecond of thought about it," Romney told a rally. "It would've been an immediate laugh."

In March 2004, McCain had said: "John Kerry is a close friend of mine. We have been friends for years. Obviously I would entertain it."

But he added: "I foresee no scenario where that would happen."

On Monday, McCain laughed off the criticism, accusing Romney of lacking conservative credentials, citing the near-universal health care legislation Romney signed with Massachusetts' Democratic-controlled legislature in 2006.

"Certainly his big government mandated health care system for the state of Massachusetts ... is not conservative," McCain told reporters at a shipyard in Jacksonville.

"He is consistent in that he has flip-flopped on every single major issue. As the liberal governor of Massachusetts, he raised taxes by $730 million."

MCCAIN GAINS IN POLL

A Reuters/C-SPAN/Zogby poll showed McCain gaining 3 points after winning the backing of Florida Gov. Charlie Crist on Saturday. The shift broke a tie at 30 percent and pushed McCain into a 33 percent to 30 percent lead over Romney.

Former New York Mayor Rudy Giuliani, who pulled out of early voting states to concentrate on Florida, was a distant third at 14 percent in the poll, which had a margin of error of 3.4 percentage points.

Romney, a multimillionaire former venture capitalist, has cast himself as a Washington outsider who would use his 25 years of business experience to pull the U.S. economy from the brink of recession and fix a "broken Washington."

Some critics question the economic effectiveness of Romney's 2003-2007 term as Massachusetts governor. While closing a budget deficit, he also generated more than $500 million by raising fees and closing corporate tax loopholes -- actions considered tax increases by some businesses.

Opponents have also attacked his credibility, pointing to his shifting positions on issues ranging from abortion to gay rights and gun control.

McCain, a 71-year-old former Vietnam prisoner of war and four-term senator who would be the oldest person elected to a first presidential term, has argued that his experience in the Navy and 25 years of leadership in Congress would make him a better commander in chief at a time of security threats. (Editing by John O'Callaghan) (For more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at http://blogs.reuters.com/trail08/)

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