Sen. Herb Kohl (D-Wis.) won’t run in 2012, leaving yet another Democratic seat open and in play. The Cook Report notes: “Democratic Sen. Herb Kohl’s announcement today that he will not seek a fifth term puts another Democratic seat in play in a state where Republicans made significant gains in 2010 and one that both states will target in 2012. The race moves to the Toss Up column, bringing the total number of Democratic-held seats in that column to eight, compared to just two Republican-held seats that are rated as Toss Ups.”

Kohl was by no means a shoo-in, but he did have the advantage of being known. The Democratic primary could get crowded, according to Jennifer Duffy of the Cook Report: “Potential Democratic candidates include Barrett, former Sen. Russ Feingold, U.S. Reps. Ron Kind and Tammy Baldwin, and former Gov. Jim Doyle. The dispute over collective bargaining could propel Barrett into the race and he could benefit from support from voters unhappy with Walker on the issue. At the same time, he’s run statewide before and may have only one more chance with voters; the conventional wisdom is that he wants another shot at Walker in 2014.”

Rep. Paul Ryan (R-Wis.) will be on everyone’s mind to replace Kohl, but many think he won't run. After all, he passed up a chance in 2010 to run against Russ Feingold. Republicans also like the state attorney general, J.B. Van Hollen, one of the few Republicans to win statewide in 2006. In 2010 he was reelected with 58 percent of the vote. Unlike other contenders, he could run without risking his current job. Duffy also mentions former congressman Mark Neumann but notes that he “made an unsuccessful bid against Feingold in 1998 and ran for the GOP gubernatorial nomination last year. He got 38 percent after running a dreadful campaign.” A GOP operative with whom I spoke also says “worth mentioning” is wealthy businessman Tim Michels, who ran in 2004.

For now, the betting on the Hill is that the Senate will flip to a Republican majority. I won’t say “control” because 60 is well out of reach. Nevertheless, with Wisconsin, Virginia, Florida, North Dakota, Pennsylvania, Missouri, Montana, New Mexico, West Virginia and Nebraska as potential pick-ups, the Republicans could wind up in the mid-50s. Much will defend, of course, on the nominees and whether the GOP has a strong candidate at the top of the ticket.