Russ Feingold has a good approval rating and a double digit lead over his seemingly strongest potential 2010 opponent, who doesn't appear likely to run anyway.
53% of Wisconsin voters approve of the job Feingold is doing with just 36% disapproving. Of the 30 or so Senators PPP has measured approval ratings for in the last year that puts Feingold in the top ten for popularity.
He has 80% approval from Democrats, 20% from Republicans, and a 50/37 spread with independents.
In a hypothetical contest against Congressman Paul Ryan, Feingold leads 51-39. That 12 point lead is identical to his winning margin in 2004 and speaks to the fact that while he has enough detractors that he's not likely to top 60% unless Republicans don't make any effort at all, his position is strong enough that he won't be a top GOP target next year either.
Barack Obama has a 55% approval rating, highlighted by a 65% approval mark from the voters under 30 who were so pivotal to his surprisingly lopsided victory in the state's primary last year. Herb Kohl has a solid 50/36 approval breakdown as well.
Full results here.