Sen. Sam Brownback (R-Kan.) won't seek reelection in 2010 and will explore a run for governor.

He plans to announce his retirement from the Senate on Thursday, reports CNN. Brownback joined the Senate in 1996, succeeding former Senate Majority Leader Bob Dole (R). He has long been a supporter of term limits, and he had pledged to serve only two full terms.

Brownback the second Republican senator, after Mel Martinez (Fla.), to decide against seeking reelection this cycle.

Possible candidates for his seat include GOP Reps. Todd Tiahrt and Jerry Moran and Kansas Gov. Kathleen Sebelius (D).