By Saundra Amrhein

ST. PETERSBURG, Fla., Nov 4 (Reuters) - Former Florida Governor Charlie Crist, who quit the Republican Party and became a Democrat, launched a campaign on Monday to win back his old job, setting the stage for what is expected to be one of the most expensive governor's races in the country next year.

At the rally where he formally announced his campaign before a crowd of supporters, Crist criticized current Republican Governor Rick Scott.

"It's not a sin to reach across the aisle. It's your job to work together. So yes, I'm running as a Democrat and I'm proud of it," Crist said.

Crist, a 57-year-old lawyer, was heavily criticized by fellow Republicans after accepting stimulus money for his state from President Barack Obama and hugging the president on camera. Crist ran for the U.S. Senate as an Independent and lost. He switched parties last year and openly campaigned for Obama's re-election.

Opinion surveys in recent months show Crist holding a lead over Scott in any potential matchup for the gubernatorial race in Florida, a perennial swing state and the fourth most populous in the United States.

A win by a Democrat in the Florida governor's race would be the party's first in more than a decade, and could have implications for the 2016 U.S. presidential race. Republicans control every statehouse across the South.

A millionaire former healthcare executive, Scott poured some $85 million into his 2010 election campaign, according to campaign records. The amount more than tripled what his predecessor Crist spent during his 2006 campaign.

Scott, who won office with support from the conservative Tea Party wing of the Republican party, has struggled with low approval ratings and faced criticism over his hardline stance on issues including healthcare reform, education and voting rights.

Crist, who served as governor from 2007 to 2011, portrayed Scott in his speech as an out-of-touch elitist and emphasized his own record on education, the environment and voting rights.

"Under this administration, big business, big lobbies and big contributers are the winners," Crist said.

Crist pledged that if elected he would work for several initiatives including giving education incentives to graduate students in the medical, science and engineering fields if they stay in Florida after graduation. He also vowed to boost state investment in roads, bridges and ports.

"Common sense versus nonsense," said Crist, who is expected to face a challenge for the Democratic nomination from former state Senator Nan Rich.

Republicans have repeatedly criticized Crist, who became an Independent after initially running for the Republican nomination for a Senate seat in 2010 against the ultimate winner, Marco Rubio.