The Cook Political Report, already releasing some assessments of the 2012 election cycle, says Herb Kohl is a solid Democrat.
Just remember that ratings sites like Cook called Feingold solid as recently as three or four months before the election. A key difference, however, is that Kohl's wealth makes Ron Johnson look like a pauper. If he really wants another six years in the Senate, he could spend tens of millions in that effort. The existence of that wealth scares of Republican opponents.
Kohl is not a terribly inspiring figure on the stump, but he has not had a close election since he first won in 1988. In fact, he won by 18 points in 1994, a Republican wave year.
Furthermore, Kohl will have the advantage of running during a presidential year, when there is less of a risk of an "enthusiasm gap" dooming the Democrats. Just as Feingold won by double-digits in 2004, it is likely that Kohl could win even if Obama loses Wisconsin.
Nevertheless, there are plenty of Democrats who would like to see him throw in the towel. Some of them want Feingold to run for the seat, while others simply want new blood and a stronger presence in the seat. Tammy Baldwin and Ron Kind are both potential candidates. The $600,000 Baldwin has not spent in recent campaigns will serve as leverage in gaining support for a primary candidacy. As for Ron, well, that guy's just a stud.
If 2012 is a bad or average year for Democrats, Wisconsin Dems will want the guy with the best chance of winning, inspirational or not. If Sarah Palin is the Republican candidate and 2012 turns into a Democratic landslide, you'll definitely hear some disappointment from progressives about the missed opportunity to replace Kohl.