MILWAUKEE (AP) — Wisconsin Democratic U.S. Sen. Herb Kohl says he will not seek a fifth term.The announcement by the 76-year-old Kohl is a blow to Democrats who will now have to defend another open seat against Republicans in a swing state. Kohl, also owner of the NBA Milwaukee Bucks, is the fifth Democratic senator to announce his retirement ahead of the 2012 election.Kohl is also one of only two Democrats who hold statewide office in Wisconsin. Republicans are coming off a year in which they defeated Russ Feingold, reclaimed the governor's office and both houses of the Legislature and picked up two seats in Congress.Kohl would have been a formidable candidate, given his personal wealth and past success. He won re-election in 2006 with 67 percent of the vote.