Updated: 1:40 p.m.

Wisconsin Sen. Herb Kohl (D) announced Friday that he will not seek a fifth term, opening a vulnerable seat that could help Republicans in their pursuit of the majority.

“So even though I continue to love this job, I have decided that the time has come to give someone else the opportunity to serve,” Kohl said at a press conference in Milwaukee, according to a press release. ”Therefore, I am announcing today that I will not seek another term as your Senator.”

Kohl, 76, had loaned his campaign $1 million in the fourth quarter but had been keeping a low profile in the battleground state. News of Kohl’s retirement was first reported by the Washington Post.

Former Sen. Russ Feingold (D-Wis.), who served with Kohl from 1993 through 2010 and could be a candidate to replace him, said, “Senator Kohl has served the state with honor. He will be remembered for his advocacy for our state’s dairy farmers, his work on behalf of children and his keen understanding of our state’s business community.  I sincerely wish him all the best.

Feingold saw now-Sen. Ron Johnson (R), a wealthy businessman with no political experience, come out of nowhere and pull off a 5-point win in 2010.

So far, the focus in Wisconsin has been squarely on Gov. Scott Walker (R) and the state Legislature, with a movement toward recall elections for state Senators and possibly even Walker. What happens there could also play a big role in what the Senate race looks like a year from now.

While there’s no doubt the move is a blow for Democrats, who already are facing the prospect of defending 23 seats to the GOP’s 10 next year, Democratic strategists insist they’re confident they can win the seat.

“There is no place in the country where Republicans have overplayed their hand like they have in Wisconsin,” one Democratic campaign strategist said. “Over the last three years, Democrats have amassed a 10-point [registration] advantage. Democrats also have a very deep bench in the state.”

As potential candidates, Democrats are floating names such as Tom Barrett, the Milwaukee mayor who ran for governor last year and narrowly lost to Walker. Progressives like seven-term Rep. Tammy Baldwin, who has been a prominent figure in the state’s labor clash and reported $709,000 in her campaign account at the end of March. Sources close to Baldwin say she is likely to run.

Eight-term Rep. Ron Kind is also a Democrat to watch, as is Feingold.

For Republicans, Rep. Paul Ryan (R) is the obvious choice to step forward and run for the open seat, but he just took the gavel of the House Budget Committee, giving him more influence than a freshman Senator. Ryan will make a statement Friday afternoon on the topic.

A more likely candidate could be state Attorney General J.B. Van Hollen, who first won statewide in 2006, a tough year for Democrats. He was re-elected last year with 58 percent of the vote and would not need to give up his post to run for the seat.

Other potential Republican candidates include two previous Senate candidates — former Rep. Mark Neumann, who ran for governor last year and lost to Feingold by 3 points in 1998, and wealthy businessman Tim Michels, who lost to Feingold by 11 points in 2004.

Democrats note that President Barack Obama competed hard there in 2008 and won the state by 14 points.

“This is a blue seat that will stay blue in a blue year in Wisconsin,” a Wisconsin-based Democratic strategist said. “It’s a presidential year. Scott Walker may be on the ballot — he’s totally activated the Democratic base.”

The National Republican Senatorial Committee had this to say:

“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.”

Kohl is the eighth Senator to retire this cycle. Here is Roll Call’s updated casualty list.

One well-placed Republican operative also suggested Republican National Committee Chairman Reince Priebus as a possible candidate.

The Republican operative, who knows Priebus and has worked extensively in Wisconsin, said he anticipates an effort in the Badger State to get Priebus to come home and bid for the Senate, but the source expressed skepticism that the RNC chairman would actually run for the seat.

Shira Toeplitz and Nathan Gonzales contributed to this report.