Wisconsin Sen. Herb Kohl (D) will retire in 2012

WASHINGTON – Democrat Herb Kohl of Wisconsin announced Friday that he will not seek reelection to a fifth term in the U.S. Senate, saying that it's "time for someone else to serve."

Kohl, 76, was first elected to the Senate in 1988, and serves as chairman of the Senate Special Committee on Aging.

The low-profile senator's retirement opens up an important seat in a state garnering lots of national attention amid the enactment of controversial anti-union laws.

Progressives are hoping that ousted Sen. Russ Feingold will run for the seat, although a spokesman said the Wisconsin Democrat would not yet comment on his plans. Other rumored Democratic candidates include Milwaukee Mayor Tom Barrett and Reps. Ron Kind and Tammy Baldwin.

On the Republican side, spectators are mentioning Rep. Paul Ryan, the chairman of the budget committee who authored the GOP's plan to phase out Medicare and cut taxes on the rich.

Though it's not certain either of the two will run, a March survey by the Democratic-affiliated Public Policy Polling found Feingold leading Ryan in a head-to-head race by 49 to 42 percent.

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