FOR THE RECORD:
The decision by Kohl, 76, had been rumored for months. He had not raised much for a 2012 campaign, though he could have tapped his personal wealth that comes from the family's eponymous department store chain.
He becomes the ninth senator -- including John Ensign, who ultimately resigned and has been replaced -- and the sixth member of the Democratic caucus to decide to leave the Senate since the start of the 112th Congress. Democrats had hoped their retirement woes had ended. The party is defending 23 seats in 2012, six of which are now open, hoping to keep what is currently a 53-47 majority.
Wisconsin is one of a host of blue-leaning states that turned sharply toward the Republicans in the 2010 election. Democratic Sen. Russ Feingold lost his re-election bid that year to Republican Ron Johnson, while Scott Walker won the governorship.
A Democratic source says the party is confident it can hold the seat, however, in part because of the controversy Walker sparked in his bid to reshape the state's collective-bargaining laws.
"There is no place in the country where Republicans have overplayed their hand like they have in Wisconsin," said the source, who requested anonymity to candidly discuss strategy.
Potential candidates include Tom Barrett, who lost to Walker, and current Reps. Ron Kind and Tammy Baldwin. It's unclear whether Feingold might also be interested in returning to Congress.
The Democratic nominee will benefit from the fact that President Obama is expected to fiercely contest for the state's 10 electoral votes.
The Republican field remains wide open; Kohl's decision will only invite more aspirants. Rep. Paul Ryan, chairman of the House Budget Committee, is at the top of some party strategists' wish list.
"We didn't see it coming," Ryan said Friday. "Right now it's Herb Kohl's day, and we should thank Herb for his service."
State Atty. General J. B. Van Hollen has a record of winning statewide.
"Senator Kohl's retirement, just like his Democrat colleagues who stepped aside before him, immediately presents another key opportunity for Senate Republicans next year," said National Republican Senatorial Committee spokesman Brian Walsh.
Kohl, who also owns the Milwaukee Bucks, is serving his fourth term. His decision means that the five most senior Democrats facing reelection have all decided to step down.
Lisa Mascaro contributed to this report.