Herb Kohl, Democratic senator from Wisconsin, will retire instead of seeking reelection in 2012. 

Kohl’s retirement raises the possibility of two interesting candidates for the open seat: Russ Feingold, the civil libertarian liberal Democrat who lost his Senate seat to Ron Johnson in the midterms, and Budget Committee chairman Paul Ryan. 

Ryan already seems open to the idea of a Senate campaign, announcing that he will “discuss this news with my family and supporters before making any decision about how I’m best able to serve my employers in the First Congressional District, our state and nation.” There are two complicating factors to a Ryan campaign, though: first, he has powers and privileges as Budget chief that he wouldn’t have as a junior senator, and second, any involvement with the 2012 Republican presidential ticket would likely preclude a Senate run.