Amid a rash of retirements from the U.S. Senate, Florida's Bill Nelson (D) is seeking another term.
Nelson told reporters that he's running at the annual legislative meeting sponsored by the Associated Press in Tallahassee on Wednesday.
Republicans are already starting to line up against the Orlando-area Democrat, including U.S. Rep. Connie Mack and Senate President Mike Haridopolos. George LeMieux (R), who filled the seat temporarily when Mel Martinez (R) stepped down early, is also expected to jump in.
A competitive Republican challenge to Nelson could help the party mobilize voters in the 2012 presidential election. Nelson - the lone Democrat elected statewide in the battleground state -- hasn't faced a strong rival since his election to the Senate in 2000.
Nelson has served in public office since his election to the Florida Legislature in 1972 and largely stuck to mainstream causes, such as advocating funding for the space program and opposing offshore oil drilling.
"I think he's more vulnerable in a primary than he is in a general,'' said Miami-based political consultant Derek Newton. "He comes from a great line of political moderates in Florida. He's no lefty, so he'd be tough to beat in a general election.''