Another Democrat has decided to retire from the Senate rather than face re-election, and this one will hurt.  Herb Kohl, currently serving his fourth term in the upper chamber, will announce that he will not seek re-election.  Not only does that force Democrats to field a new candidate in Wisconsin, a state that turned Republican in 2010, it means that they lose someone with the potential to be a self-funder as well:

Wisconsin Sen. Herb Kohl will announce his retirement today according to an informed Democratic source, a decision that sets up a competitive open seat contest in a state that has already drawn considerable national attention this year.

Kohl will make it official this morning with a noon announcement in Wisconsin, the source said. His office did not return an email seeking comment.

Democrats tell Chris Cillizza that they’re looking forward to the race because Scott Walker’s PEU reforms have “energized the party.”  Unfortunately for the party, it will also have drained union coffers at a particularly tough moment in Wisconsin politics.  It’s also worth noting that the unions with their full financial power still intact couldn’t even unseat a targeted Supreme Court justice in an off-year election.  That sounds more like bravado than cogent political analysis.

Who will Democrats tap for the nomination?  Cillizza suggests that Russ Feingold might want to make a comeback, but that would be a desperation shot.  Feingold’s hard-Left (but honest!) progressivism has clearly faded from favor everywhere but Madison and parts of Milwaukee.  Feingold lost an election to a novice last year, and it wasn’t even close.  Republicans swept the election, and this year they delivered the best budget in almost two decades, wiping out a series of Democratic billion-dollar-deficit plans.  Wisconsin might be open to a Democratic moderate, but they’ll laugh the big-spending Feingold out of the race.

Kohl apparently got the sense of the political situation in Wisconsin and got out.  If the party was really as energized as Cillizza’s Democratic sources claim, he’d be running for a fifth term.  This race already promised to be a fun watch, and now it just got even better for Republicans.

Update: National Journal notes that this is the sixth Democratic retirement in the Senate (including Joe Lieberman), to three for the GOP, which included John Ensign.  They also point out that Kohl is leading the Senate review of the AT&T/T-Mobile merger.

Update II: Steve Eggleston gives an excellent analysis of the bench on both sides in our Green Room.