PPP's first look at the Wisconsin Senate race this cycle finds Russ Feingold with a clear lead over Ron Johnson in a hypothetical rematch of their 2010 contest. Feingold gets 50% to just 41% for Johnson.
Johnson hasn't proven to be very popular during his first term in the Senate. Only 32% of voters approve of the job he's doing to 40% who disapprove. 28% of voters with no opinion about him also suggests he hasn't made a terribly strong impression on people over the last 4 years. Meanwhile Feingold is still relatively popular despite his 2010 loss. 46% of voters see him favorably to 35% with an unfavorable opinion. That makes him more popular than any other politician in the state who we looked at on this poll.
Feingold has a big initial advantage over Johnson for a couple key reasons. He is up double digits with independents, at 52/38. He's also winning 11% of the Republican vote while losing just 4% of Democrats to Johnson. It's relatively unusual to see the Democratic base that much more unified than the GOP one.
PPP also tested a Feingold/Johnson rematch on 3 separate polls during the 2013-14 election cycle. On those Feingold held advantages of 47/41, 49/42, and 52/42. So a high single digit lead for Feingold with Johnson stuck in the low 40s has proven to be a pretty consistent finding over time.