Interestingly, he notes that "never before has a U.S. senator gone home to take on his or her own party's governor in a primary. As it is, the list of senators elected governor is small; just five have made the move in the past half-century or so: Price Daniel (D-TX) in 1956, Pete Wilson (R-CA) in 1990, Dirk Kempthorne (R-ID) in 1998, Frank Murkowski (R-AK) in 2002, and Jon Corzine (D-NJ) in 2005. But none challenged an incumbent to do so, in a primary or in the general."
Senators are rarely elected to anything else, because they have to take so many stupid votes. Nobody likes senators... Obama lucked out because he had really only served a year or two in the senate before running for prez.