By Jason Szep and Tim Gaynor

JACKSONVILLE, Florida (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 February 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."