If Wisconsin had not endured enough political turmoil already this cycle, Sen. Herb Kohl’s retirement puts a formerly safe Democratic seat up for grabs.

Kohl’s retirement is a major disappointment for Democrats, who have already had five other Senate retirements this year. The Friday surprise creates the fifth competitive open seat that Democrats will have to defend in 2012, when the party was already largely expected to play defense. Senate Democrats must defend 23 seats in this cycle, compared with only 10 GOP-held seats that Republicans are trying keep.

The GOP needs just four seats to win back control of the chamber.

Wisconsin has long been a battleground state where presidential candidates will go down to the wire fighting for every last electoral vote. But over the course of the past year, the state has witnessed massive political upheaval — from weeks of overnight protesters camped out in the state Capitol, a state Supreme Court race that garnered national attention and an upcoming recall election for state lawmakers.

This competitive Senate race might include one of biggest names on Capitol Hill who is quickly becoming a national figure — Kohl’s retirement forces a tough decision for House Budget Chairman Paul Ryan. In the midst of the biggest budget battle in decades, the seven-term Republican must decide whether he will keep his current job or follow his longtime aspirations to run statewide.

“It’s not a secret that Paul Ryan has talked about this seat,” said Scott Becher, a veteran Wisconsin Republican strategist. “The downside is that he gives up a really good job in the House. Does he want to give up something that he’s worked so hard for — and the opportunity to eventually be a Speaker of the House — to be a U.S. Senator and eventually run for president?”

Ryan did nothing to tamp down speculation that he was considering a bid.

“I was surprised by Senator Kohl’s announcement and want to take some time over the next few days to discuss this news with my family and supporters before making any decision about how I’m best able to serve my employers in the First Congressional District, our state and nation,” Ryan said in a statement Friday after Kohl announced his retirement.

Kohl’s decision to step down after four terms shocked many on Capitol Hill — including the Democratic Senatorial Campaign Committee and Democrats in the Wisconsin delegation. DSCC Chairwoman Patty Murray (Wash.) told reporters last month that she did not expect any more retirements in her caucus.

Roll Call Politics moved its rating of this race from Leans Democratic to Tossup.

Democrats quickly sought to push back on the news, emphasizing a favorable political climate in the state, including statewide polls showing a generic ballot  advantage.

“Our job is now to make sure the seat remains in the hands of a Democrat, someone like Herb Kohl,” Wisconsin Democratic Party Chairman Mike Tate told reporters Friday. “We will be victorious in electing a Democrat to the United States Senate in 2012.”

However, both Democrats and Republicans boast deep benches in the Badger State, with both sides likely to encounter primary battles for the nominations.

Several Democratic candidates immediately emerged as possible successors — including former Sen. Russ Feingold, Milwaukee Mayor Tom Barrett and Reps. Tammy Baldwin and Ron Kind.

Baldwin is likely to enter the race, according to sources close to the Congresswoman.

Democratic sources say Kind and Barrett are looking at bids but both declined to address the Senate race in statements released by their respective offices.

Within hours of Kohl’s announcement, Democracy for America was already promoting Feingold’s candidacy, announcing a draft campaign.

But Feingold, who lost re-election last year to now-Sen. Ron Johnson (R), might not be ready to jump in yet. His former chief of staff, Mary Irvine, told the Milwaukee Journal-Sentinel that “running for office in 2012 is not something Russ is eager to do or has been planning to do.”

“He will come to a decision in the coming months, after consulting with family and friends and people in Wisconsin,” she added.

Regardless of whether Ryan runs for Senate, there’s no shortage of GOP candidates who could run for the seat.

National Republicans were quick to tout state Attorney General J.B. Van Hollen, who won re-election by large margins in 2006 when Democrats swept offices across the state. Van Hollen would not have to give up his current job to run for the Senate seat — a major benefit for Republicans if he is seriously considering a run, especially because many in the state do not view him as a risk-taker.

Republican businessman Tim Michels, who unsuccessfully ran for Senate in 2004, confirmed his interest in the race in an interview with Roll Call.

“Right now, I’m just working my tail off,” Michels said. “It’s something that I’m sure a lot of people will consider, myself included.”

Ex-Rep. Mark Neumann (R) is viewed as a likely candidate. He was already making the rounds in Washington, D.C., about a Senate run before Kohl made his decision. Neumann lost to Feingold in the 1998 Senate race and lost to Scott Walker in the 2010 gubernatorial primary — and he’s personally wealthy.

“My phones have been ringing off the hook today with calls of encouragement,” Neumann said in a statement.

Former Rep. Mark Green (R) is also a potential candidate. He would have to move back to Wisconsin from D.C., where he’s head of the Malaria Policy Center, a project of Malaria No More. Green served four terms in the House, representing the Green Bay area, before running unsuccessfully for governor against Democrat Jim Doyle in 2004.

State Senate Majority Leader Scott Fitzgerald (R) has been mentioned, but with the ongoing recall elections, it would be quite a time for him to abandon the ship.

At least one Republican operative who has worked extensively in Wisconsin anticipated a draft campaign for Republican National Committee Chairman Reince Priebus — a former Wisconsin GOP chairman. But the source cautioned that the newly elected GOP boss would likely keep his current post.

However, Priebus has vowed to make the state competitive in 2012, and this winter’s budget battle that targeted organized labor has energized grass-roots activists on both sides. Democrats say they are confident they can win state Senate recall elections this summer and flip the three seats necessary to take back control.

Also on the horizon is an expected push to try to recall Walker. Democrats intend to pursue a recall election, which also might attract Feingold, after Walker has completed one year in office.

Nathan L. Gonzales contributed to this report.