Wisconsin Sen. Herb Kohl is retiring, making him the sixth senator in the Democrats' caucus to opt out of running for reelection in 2012, National Journal's Sean Sullivan reports. Wisconsin is expected to be a big battleground next election, as evidenced by the weeks-long battle over Gov. Scott Walker's push to curb public sector union power and the subsequent attempts to recall state senators involved in the dispute. Among those who are potential contenders for Kohl's seat are Rep. Paul Ryan, author of the House GOP's controversial budget bill that overhauls Medicare, and former Democratic Sen. Russ Feingold, who lost his seat in last fall's midterm elections.

Kohl was first elected in 1988. He'll give a press conference in Milwaukee at noon Friday.