Wisconsin Sen. Herb Kohl’s announcement Friday that he will retire from the Senate at the end of his term deals a major blow to Democrats’ hopes of keeping control of the upper chamber.

Senator Kohl, whose fourth term finishes next year, was considered a lock to hold the seat had he run for reelection. But the soft-spoken former department-store magnate and owner of the Milwaukee Bucks decided to call it a day, politically.

“Even though I continue to love this job, I have decided that the time has come to give someone else the opportunity to serve,” Kohl said from his Milwaukee Senate office.

The Democrats already faced an uphill battle in their effort to retain control of the Senate in 2012, with 23 Democratic-held seats and only 10 held by Republicans up for election. Kohl is the sixth Democratic-aligned senator to say he will not run for reelection, compared with only two Republicans.

The Republicans will need a net gain of four seats to seize control of the Senate, with a current lineup of 53 Democrats and 47 Republicans. According to the nonpartisan Cook Political Report, eight of the 23 Democratic-held seats are considered tossups, while only two of the 10 GOP-held seats are.

The open Wisconsin seat sets up a test for the state’s resurgent Republican Party. Last November, the GOP won the governorship, a Democratic-held Senate seat, and five of the state’s eight congressional districts, including two that had been Democratic-held. The big clash between Republican Gov. Scott Walker and organized labor earlier this year has heightened partisan tensions in the state, which will surely play out in what is expected to be a highly competitive Senate race.

Among the potential contenders: For the Democrats, former Sen. Russ Feingold, who lost last year to tea party-backed businessman Ron Johnson; Milwaukee Mayor Tom Barrett, who lost to Walker in the governor’s race; and Reps. Ron Kind and Tammy Baldwin.

For the Republicans, one possibility is former congressman and former gubernatorial candidate Mark Neumann, who was already considering a run before Kohl’s announcement. There’s also seven-term Rep. Paul Ryan, but given his pivotal role in the House as chairman of the Budget Committee, he may well stay put in the House.