Democrats usually do slightly better on election day than they do in the polls (even if they end up losing anyway).
Walker did overperform some polls in the 2012 recall (PPP's final poll of the 2012 recall had Walker up 3, and he ultimately won by 7), so Democrats overperforming their poll numbers might not apply to this race. On the other hand, Walker underperformed in 2010 compared to the polls. My point is that the picture isn't very clear regarding which side will overperform their poll numbers on November 4.
PPP has an iffy track record in Wisconsin. They were actually the most pro-Ron Johnson poll in 2010.
