Sen. Herb Kohl (D-Wis.) will announce Friday that he won’t seek reelection in 2012, a Democratic source confirmed.

Kohl, the quiet Wisconsin senator and owner of the NBA’s Milwaukee Bucks, will depart from the Senate after four terms, setting up what could be a tough battle for his seat.

Kohl becomes the sixth senator in the Democrats’ corner to decline reelection next year; Daniel Akaka (Hawaii), Jeff Bingaman (N.M.), Kent Conrad (N.D.) and Jim Webb (Va.) have said they’ll retire at the end of this term, as will Sen. Joe Lieberman (Conn.), an Independent who caucuses with Democrats.

{mosads}Democrats lost a Senate seat in Wisconsin in 2010 when Russ Feingold was defeated by Sen. Ron Johnson (R). 

Democrats are defending 21 Senate seats next year, compared to 10 for Republicans. Democrats are also defending two seats held by Independents who caucus with Democrats.

Some immediate possible candidates could include Feingold, a liberal icon who has stayed politically active through his new Progressives United political action committee. Republicans might try to lure Rep. Paul Ryan (R), the House Budget Committee chairman who hails from a competitive district, to jump into the race.

GOP candidates enjoyed a great deal of success in Wisconsin last fall, picking up not only Feingold’s seat but the governor’s mansion and two House seats. Democrats hope to have a chance at winning back some of those seats after the heated battle over a collective bargaining reform bill Gov. Scott Walker (R) forced through the State Legislature.

Sen. Patty Murray (Wash.), the chairwoman of the Democratic Senatorial Campaign Committee (DSCC), had said last month that she expected no more retirements by Democratic incumbents. She’s been pushing possible retirees to make up their minds about running again as quickly as possible.

There has been some chatter that Kohl, 76, would not seek a fifth term.

The wealthy Kohl loaned his campaign account $1 million at the start of the year. That move was seen as a good sign for Democrats, but he’d since remained quiet about his intentions.

Liberal groups may work to aggressively court Rep. Tammy Baldwin (D) to consider entering the race. Baldwin, a seven-term liberal Democrat, could even excite party activists more than Feingold’s reentry into politics.

Milwaukee Mayor Tom Barrett, who made an unsuccessful run for governor last year, and Rep. Ron Kind (D-Wis.) are also considered contenders to succeed Kohl.

This story was last updated at 11:34 a.m.