As expected, Sen. Sam Brownback (R-Kans.) will announce tomorrow that he plans to retire from the Senate when his term ends in 2010, CNN reports. The second-term senator has been eyeing a run for governor for years now and will likely file official papers to do so next month.

Current Kansas Gov. Kathleen Sebelius, a Democrat, will be term-limited in 2010, and many Democrats hope she'll run for the vacant Senate seat. However, history is not on her side -- according to the Almanac of American Politics, Kansas has not elected a Democratic senator since 1932, which is the longest streak of any state.

Brownback becomes the second Republican senator to announce his retirement this month. Sen. Mel Martinez (R-Fla.), in his first term, announced two weeks ago that he would not be running for re-election in 2010.

Republicans already had more seats up for election in 2010 than Democrats, so these openings could spell more trouble for the party. However, with President-elect Obama's pick of Sen. Ken Salazar (D-Colo.) for Interior Secretary, four Democrats will now leave the Senate for the executive branch. Of the four, Salazar's seat is likely the most vulnerable for Republican takeover in two years.