Chris Cilizza reports that Wisconsin Democrat senator Herb Kohl plans to call it quits after 2012:

Wisconsin Sen. Herb Kohl will announce his retirement today according to an informed Democratic source, a decision that sets up a competitive open seat contest in a state that has already drawn considerable national attention this year.

Kohl will make it official this morning with a noon announcement in Wisconsin, the source said. His office did not return an email seeking comment.

Kohl's retirement would mean that five Democratic senators are leaving open seats in 2012 (Daniel Akaka of Hawaii, Jeff Bingaman of New Mexico, Kent Conrad of North Dakota, and Jim Webb of Virginia), along with Joe Lieberman of Connecticut, the independent who caucuses with the Democrats. Only two Republican senators, Jon Kyl of Arizona and Kay Bailey Hutchison of Texas, are retiring next year.

A number of Wisconsin Democrats could get into the race to replace Kohl, including Russ Feingold, the liberal senator who was ousted last fall by Republican Ron Johnson in one of the midterm election's biggest upsets.

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