Herb Kohl, one of the more enigmatic members of the Senate, will not seek re-election, according to Democratic sources in the state. He has served in the Senate since 1989.

Kohl, 76, the wealthy owner of the eponymous discount clothing franchise and the chairman of the Senate Committee on Aging, has made less public comments during his Senate career than perhaps anyone in recent memory. He’s generally a mainstream Democratic vote, but I cannot think of one cause he’s championed in his entire career. In several thousand posts on this site, I’ve mentioned him in passing exactly five times.

Kohl becomes the ninth Senator to choose to retire, ensuring another election cycle with massive turnover. Wisconsin elected Republican Ron Johnson to the Senate last year, but the unrest in the state over Scott Walker’s attack on unions has emboldened Democrats and deepened their bench. Russ Feingold, who was defeated by Johnson in 2010, immediately becomes a likely candidate to return to the Senate in a Presidential election year and under more favorable terms. The Milwaukee Journal-Sentinel draws up a short list on both sides:

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.

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.

That quote came well before Feingold started Progressives United and before Kohl decided to retire.

A Feingold/Ryan matchup would be a real battle of competing ideologies between two Wisconsinites from the same city, Janesville. Ryan has been pounded for his budget which would end Medicare and cripple Medicaid. The group Faith in Public Life just put out an ad narrated by a Catholic priest calling the budget “not pro-life.”

There’s also the prospect of a recall election for Governor Scott Walker next year. Wisconsin Democrats seem to have a deep bench and the upper hand in the state consciousness right now. The Senate open seat could be a showcase for this.