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Kohl won't seek re-election in 2012

A high-placed Democratic source tells First Read that Wisconsin Sen. Herb Kohl (D) will announce he won't seek re-election in 2012.

Kohl, a four-term senator who was first elected in 1988, is holding a press conference in Milwaukee at noon ET.

Kohl's announcement means that we'll see yet another competitive open-seat Senate race in a presidential battleground state next year, joining Nevada and Virginia.