Wisconsin Sen. Herb Kohl announced his retirement Friday, a decision that sets up a competitive open seat contest in a state that has already drawn considerable national attention this year.

“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, who is in his fourth term and 76 years old, said in a noon speech in Milwaukee.

Kohl is the sixth Democratic-aligned Senator to step aside in advance of the 2012 election. Two Republicans aren’t running for reelection next year.

“Today, I called Senator Herb Kohl to thank him for his remarkable career in public service,” President Obama said in a statement. “During his 23 years in the United States Senate, Herb’s invaluable perspective as the long-time head of a family-owned business made him an unwavering voice for working families, small business owners, and seniors.”

Democratic strategists insisted that they will field a top-tier candidate in the open seat, noting that the fight between Gov. Scott Walker (R) and national unions over collective bargaining in the state earlier this year has energized the party.

The Democratic Senatorial Campaign Committee was surprised by Kohl’s decision. "Our expectation is that he'll be the last retirement," executive director Guy Cecil told National Journal.

Reps. Ron Kind, Tammy Baldwin and Steve Kagen are leaning heavily towards running, sources say. Former Sen. Russ Feingold and former Rep. Tom Barrett, who ran lost to Walker last November, have also been mentioned.

“Senator Kohl has served the state with honor,” Feingold said in a statement. “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.” Democracy for America, the political action committee founded by former Vermont governor Howard Dean, has started a petition to draft Feingold for the seat.

National Republican Senatorial Committee spokesman Brian Walsh said in a statement that Kohl’s retirement “immediately presents another key opportunity for Senate Republicans next year.”

For Republicans, the obvious name is Rep. Paul Ryan, the architect of House Republicans’ controversial budget plan. It’s not clear whether Ryan wants to leave his perch as chairman of the House Budget Commtitee to make a statewide run, however. Ryan said in a statement that he was “surprised” by the announcement and is discussing the news with family and supporters.

Other GOP names being mentioned include state Attorney General J.B. Van Hollen, wealthy businessman Tim Michels and former Rep. Mark Neumann. Neumann put out a statement saying he’s “seriously considering” the race and that “my phones have been ringing off the hook today with calls of encouragement.”