With Kohl's retirement, who will run for Wisconsin's Senate seat?

Just coming across is the news that Wisconsin Senator Herb Kohl (D) will announce his retirement. His decision forces the Democrats to defend yet another Senate seat next year, making their hold on the institution even more precarious.

Had he decided to run again, Kohl would have almost certainly been easily re-elected. After a close first election in 1988, Kohl has never received less than 58% of the vote. His ability to self-finance has always been cited as a main reason for his prowess, but there are other reasons for his strong showings, including a stellar statewide casework operation and his low key, yet approachable, demeanor. While a lot of Wisconsinites may not like the way he votes, they tend to like him personally.