I came across this December 14th, 2010 article at the Journal Sentinel online:
The highlights: Kohl leads in hypothetical match-ups with Republicans Tommy Thompson, Paul Ryan and J.B. Van Hollen … Kohl’s approval rating is 50%, his disapproval rating 35%.
Russ Feingold, who lost his 2010 re-election bid, also leads the same three Republicans in hypothetical 2012 match-ups. Public Policy Polling apparently included Feingold in its survey to account for a scenario in which Kohl decides against a fifth term and Feingold runs in his place. Kohl is still expected by many Democrats to run again.
Feingold’s approval rating is the same as Kohl’s (50%) but his disapproval rating is higher (43%).
The pollster also notes the seeming oddity of Feingold performing well when he just lost to Republican Ron Johnson:
The pollster's explanation is that this new poll is not a survey of the 2010 mid-term electorate, whose makeup was atypically conservative. It's a survey of a different and broader population of people who have voted at least once in a recent major election, including the high-turnout presidential elections of 2004 and 2008. That would presumably help explain how Feingold could have a 54% approval rating among independents in this new statewide poll after losing (according to exit polls) self-identified independent voters this past fall by 13 points. The numbers can be found here.