Crist, 52, instantly becomes the front-runner in the Senate race. He has maintained approval ratings in the high 60-percent range despite the state's gloomy economy, budget cuts, a high foreclosure rate and the highest unemployment level since 1975.
That popularity is credited to an unwavering optimism, bipartisan attitude and the projection of a sense that he cares.
The governor, whose ability to charm people is sometimes compared to Bill Clinton's, was on the list of possible 2008 GOP vice presidential candidates.
Sen. John Cornyn, head of the National Republican Senatorial Committee, quickly endorsed Crist. Senate Republicans are looking at the likelihood that the Democrats will hold a 60-seat majority in the Senate that can overcome GOP filibusters and help pass President Barack Obama's legislative agenda.
Democrats and independents hold 59 seats and would reach 60 if Al Franken wins a marathon recount in Minnesota.
Crist's aion to run for governor. Agriculture Commissioner Charles Bronson, who under Florida law can't seek a third term, has also said he will get in the race.
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