NOTE: An updated version of this story can now be found here.

U.S. Sen. Herb Kohl (D-Wis.) said Friday that he would not run for a fifth term in the U.S. Senate.

Kohl, who is 76, made the announcement at a news conference in his Senate office in Milwaukee. His departure from politics is certain to set off a wild scramble among both Democrats and Republicans to run for the seat he has held since 1989.

Kohl said. "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."

As he spoke, Kohl was surrounded by nearly two dozen staffers from the Milwaukee Bucks, as well as members of his Senate staff.

"The office doesn't belong to me. It belongs to the people of Wisconsin and there is something to be said for not staying in office too long," he said. "I've always believed that it is better to leave a job too early than a little too late, and hat's how I feel today."

Kohl said his health was good, and he had no intention of retiring from public life.

"I need to be busy every day," Kohl said.

Kohl said he had deliberated on whether to run again or not for a few weeks. Asked if he would have won had he decided to seek re-election to a fifth term, Kohl said, 'Oh, sure."

Asked why he believed he would have won, Kohl said it was because he had never been defeated.

Kohl then elaborated on the answer. He said he believed that voters in Wisconsin had come to understand the way he represented the state. He described himself as a person who is reasonable, experienced, balanced and who represents the "diversity of our interests."

Kohl described the current political landscape in Wisconsin as "toxic," but said it did not factor in his decision.

He said public officials should try to bring people together, "rather than tear them apart.

"To me, that describes public officials. We've gotten off the track, clearly," Kohl said.

Kohl’s decision serves as a political blow to Democrats, adding one more seat to a growing number the party has to defend in 2012. Republicans have never come close to defeating Kohl since he was first elected in 1988, and given his political track record and his personal wealth, he would start as a heavy favorite to win re-election next year.

Democrats have to defend 23 U.S. Senate seats next year, while Republicans have to defend only 10. While most of those seats are in states that President Barack Obama won handily in 2008, the numbers give the GOP far more opportunities on paper for pickups than Democrats have, and put control of the Senate in play.

Kohl's retirement is the latest in a series of planned departures by Democratic senators including Jim Webb of Virginia, Kent Conrad of North Dakota and Jeff Bingaman of New Mexico.

Kohl’s decision not only has implications for Democrats nationally, but would have huge consequences for Wisconsin politics, creating the kind of rare political opening that can scramble careers.

It has been 23 years since the last open-seat Senate election in Wisconsin, when Democrat Bill Proxmire retired and Kohl beat Tony Earl and Ed Garvey and Doug La Follette for the Democratic nomination and Republican Susan Engeleiter in the general election.

Potential Senate candidates on the GOP side include House budget chair Paul Ryan, state Attorney General J.B. Van Hollen and Mark Neumann. On the Democratic side, candidates would include ex-senator Russ Feingold, and U.S. House members Ron Kind of La Crosse and Tammy Baldwin of Madison and Milwaukee Mayor Tom Barrett.

Feingold was defeated last fall by Ron Johnson after serving three terms in the U.S. Senate. In an interview in Washington one month ago, Feingold was asked about the Kohl seat, and said:

“I assume he’ll run. I hope he’ll run. I want him to run. I think he’s an important person to provide balance for us in Wisconsin at this point . .. I believe this is going to be a good election for the President and I think for Sen. Kohl. I think any Republican who thinks this is going to be 2010 all over again has got another (thing) coming.”

Feingold said of his own political future: “My plate is very full . .. I have no plans to run for public office.” He is writing a book and teaching at Marquette University Law School. But that was before Kohl had announced his plans, and Feingold would automatically become a leading contender for the Democratic nomination for Senate should decide be interested.

The heads of the state Assembly and Senate might jump into the fray. John Jagler, a spokesman for Assembly Speaker Jeff Fitzgerald (R-Horicon), said his boss is interested. “If Paul Ryan doesn’t run, he would definitely consider a run,” Jagler said.

According to a source close to Sen. Scott Fitzgerald (R-Juneau), the state Senate majority leader is exploring a possible run.

As for Ryan, he has in the past dismissed the idea of running against Kohl. His national profile is so high that he has not only been mentioned as a potential statewide candidate but also as a potential vice-presidential pick in 2012. But even though the Senate is traditionally a big step up from the House, Ryan has carved out a uniquely influential national role in his party as House budget chair. Even if he ran for Senate and won, he would be trading that role to be a junior Senator, and possibly a junior Senator in the minority, should Democrats hold the Senate next year.

Brian Walsh, a spokesman for the National Republican Senatorial Committee (NRSC), said in a statement that "Senator Kohl’s retirement, just like his Democrat colleagues who stepped aside before him, immediately presents another key opportunity for Senate Republicans next year. It also further dilutes the ability of national Democrats to go on offense, while they fight to maintain their dwindling Senate majority.

“As Russ Feingold discovered last year, whomever the Democrats eventually nominate, he or she will have a very tough time selling Wisconsinites on the Washington Democrats’ agenda of reckless deficit spending, massive debt, and job-killing tax hikes. Wisconsin continues to be a prime pickup opportunity for Senate Republicans in 2012.”

Kohl serves on the Senate Appropriations Committee, Judiciary Committee, Banking Committee, and the Special Committee on Aging, which he serves as chairman.

Kohl also is the chairman of the Agriculture Appropriations Subcommittee, which has jurisdiction over the budgets of U.S. Department of Agriculture, the Food and Drug Administration. And he is chairman of the Judiciary's Subcommittee on Antitrust, Competition Policy and Consumer Rights.

Former House judiciary chair Jim Sensenbrenner (R-WI) praised Kohl's service. "While Senator Kohl and I have disagreed on many policy issues over the years, he has always served Wisconsinites with honor and integrity," he said.

A native of Milwaukee, Kohl has a bachelor's degree from the University of Wisconsin-Madison and a master's degree in business administration from Harvard.

Before turning to politics, Kohl helped build the family-owned Kohl's grocery chain and department stores.

In 1985, he bought the Milwaukee Bucks.

Kohl also has been heavily involved in charitable efforts. In 1990, he established the Herb Kohl Educational Foundation Achievement Award Program, which provides annual grants totaling $400,000 to 200 graduating high school seniors, 100 Wisconsin teachers, and 100 schools throughout Wisconsin.

On Friday morning, as word spread that he would not seek re-election, Kohl said he had received a phone call from Obama, wishing him well.

Recently, Kohl said he was at the White House and met with Obama. Obama, as always, Kohl said, talked trash with Kohl about the Bucks and his beloved Chicago Bulls.