Wisconsin Democratic Sen. Herb Kohl said Friday he's not running for a fifth term in 2012, a blow to Democrats who now will have to defend another open seat against Republicans in a swing state.

Kohl is the fifth Democratic senator to announce his retirement ahead of the 2012 election.

Republicans will see Kohl's retirement as a clear pickup opportunity. A self-funding millionaire, Kohl also is owner of the Milwaukee Bucks NBA franchise and widely popular in Wisconsin. Any candidate Democrats run is likely to need more financial support from the national party.

Democrats had thought as recently as last week that Kohl would run again.

"I've always believed it's better to leave a job a little too early than a little too late. And that's how I feel today," Kohl said during a Friday news conference in Milwaukee. "The interest and the energy I have for this job will find a new home at the conclusion of this term."

Kohl's retirement comes as a surprise to national Democrats who had hoped the millionaire would help them take a potentially competitive seat off the map. As recently as last week top Democrats in Washington talked about Kohl's re-election as if it were a matter of fact.

Possible Republican candidates wasted no time in expressing interest in running.

Wisconsin Attorney General J.B. Van Hollen, a Republican, is considering a run, according to his campaign. And Wisconsin Assembly Speaker Jeff Fitzgerald would consider running if U.S. Rep. Paul Ryan doesn't get into the race, said Fitzgerald's spokesman John Jagler.

"It is obviously early, but it is something he would look into," Jagler said.

Ryan said in a statement that he was surprised by Kohl's announcement and wanted to take some time over the next "few days" to discuss the news with his family and supporters before deciding whether to run for the seat.

Mark Neumann, a former congressman and losing GOP candidate for governor in 2010, said he is considering running, but has no timetable in mind for making a decision.

"The phone has been ringing off the hook this morning," he said.

Wisconsin Senate Majority Leader Scott Fitzgerald, the Assembly leader's brother, also is considering running.

Possible Democratic candidates include former U.S. Sen. Russ Feingold, who lost his seat to Ron Johnson in 2010, U.S. Reps. Tammy Baldwin of Madison and Ron Kind of La Crosse, and Milwaukee Mayor Tom Barrett, also a former congressman.

Kind's spokeswoman had no comment on Kind's plans. Baldwin is likely to run, said a person close to her who spoke on condition of anonymity Friday because the congresswoman wanted to keep the day focused on Kohl's retirement.

Feingold issued a statement praising Kohl but did not address whether he would run for the seat. He and Barrett did not immediately return messages.

The 76-year-old Kohl was elected to the Senate in 1988 and never has won an election with less than 52 percent of the vote. Born and raised in Milwaukee, he worked as an investor before founding the Kohl's grocery and department stores that earned him a fortune. He also is widely credited with keeping NBA basketball in the relatively small market of Milwaukee.

Kohl has been unassuming in the Senate, often focusing on local issues. He serves as chairman of the Senate Special Committee on Aging, a key post for a state with a large elderly population.

Kohl joins a group of retiring Democrats that spans the coasts, from Virginia to Hawaii, ensuring the party will be playing defense across the country.

Besides Kohl, Sens. Kent Conrad of North Dakota, Jim Webb of Virginia, Jeff Bingaman of New Mexico and Daniel Akaka of Hawaii all have announced their retirements, as has Sen. Joe Lieberman, who runs as an independent but caucuses with Democrats.

Incumbents tend to be favored to win re-election in Senate races, so the retirements give the GOP more of an opportunity. Republicans already had signaled they would run vigorous campaigns in these states.

The seat also will serve as a good barometer of the political mood in Wisconsin. Republicans swept to gains statewide in 2010, but have been put on the defensive after a long and boisterous fight over collective bargaining rights led by Gov. Scott Walker. Democrats believe the rancor that provoked will lead them to a rebound in the state in 2012.

Kohl said he spoke with President Barack Obama about his decision to retire and the president told him he would do everything he could to see that Wisconsin "does the right thing" in the next election.

Kohl's term ends Jan. 3, 2013.

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Jackson reported from Washington. Associated Press writers Scott Bauer and Todd Richmond in Madison, Wis., contributed to this report.