Sen. Russ Feingold (D-Wisc.), a progressive favorite, lost the Senate seat he held for 18 years to a Republican businessman last fall. (Linda Davidson/THE WASHINGTON POST)

Feingold was considered a potential candidate both for the Senate seat being vacated by Herb Kohl (D) and a recall campaign against Gov. Scott Walker (R).

Feingold lost his seat in fall 2010 to businessman Ron Johnson (R).

In his message, Feingold says he is “thoroughly enjoying the life of a private citizen” and simply does not want to return to public office at this time.

“I know that progressives are eager to reverse some of the outrageous policies being pursued by corporate interests at both the state and federal levels,” Feingold wrote in an email to supporters of Progressives United, the political action committee he founded in Februrary. “I am also well aware that I have a very strong standing in the polls should I choose to run again for the U.S. Senate or in a recall election for governor.”

But, he concluded, “After twenty-eight continuous years as an elected official ... I have found the past eight months to be an opportunity to look at things from a different perspective.”

Feingold will instead devote his energyto the PAC, which seeks to overturn the U.S. Supreme Court’s Citizens United decision, fight the acceptance of unlimited corporate contributions in elections, and support progressive candidates.

The former senator said he also looked forward to teaching at Marquette University Law School, finishing the book “While America Sleeps,” and spending time with his family.

With Feingold out, Rep. Tammy Baldwin (D-Wis.) is expected to announce her Senate campaign soon for Kohl’s open seat. Rep. Ron Kind (D-Wis.) is another likely contender. Neither House member has Feingold’s star power or fundraising ability, and without Feingold in the contest, the Senate race becomes a much more competitive.

National Republicans consider Kohl’s Senate seat a prime catch.

“Even Democrat[ic] polls show that this will be one of the most competitive races in the country next year,” said National Republican Senatorial Campaign Committee spokesman Chris Bond.

On the Republican side, former Wisconsin governor Tommy Thompson is the likely nominee, although he has faced continued criticism from the right for his work on health-care reform and could face a serious primary challenge. Democrats point to that possibility as a reason Republicans should still be worried.

“Republicans in Wisconsin have overplayed their hand like nowhere else in the country and now they face a potentially brutal primary in the state,” said Democratic Senatorial Campaign Committee spokesman Shripal Shah.

More on PostPolitics

Excuses, excuses on the presidential bus tour

Bachmann and the buzz factor

Elizabeth Warren launches exploratory Senate bid