I told you last month that Herb Kohl, Wisconsin’s Democratic senior Senator, hadn’t raised even a penny in the first quarter of 2011, leading to questions about whether he would run for re-election in 2012. Kohl was best known for being not known – he kept an extremely low profile in the Senate, and the best you can say of his Senate career is that, to quote Animal House, Kohl has a long tradition of existence in the Senate. Today he makes it official: he’s not running for re-election in 2012.

Kohl’s retirement sets off a scramble to identify the candidates for what will doubtless be a very high-profile race, given the recent political controversies in Wisconsin and its status as a potentially crucial 2012 swing state. The A-list Democratic candidate is recently deposed Democratic Senator Russ Feingold, who commands loyalty from the base and top-flight fundraising capability; given that Feingold was beaten in 2010 by relative unknown Ron Johnson (52%-47%) without the assistance of a scandal or other major controversy, running him sets up a pure test of whether the state’s rightward shift will endure in 2012 with the elevated turnout of a national election.

The GOP “dream” candidate is Paul Ryan, who of course is also being courted in some circles to be a presidential or vice presidential contender, but Ryan has a safe lock on an otherwise Democratic-leaning district, a powerful perch in the House and young kids, so he may pass on this race. The other main candidate who’s already jumped in is former two-term Congressman Mark Neumann, who lost a moderately close race to Feingold in 1998 (50.55%-48.40%) and finished a distant second to Scott Walker in the 2010 Gubernatorial primary.

Stay tuned.