NBC/Marist has a new poll out for Wisconsin showing Obama up 49-46. But the poll is of 1440 adults, of which 1332 self-identify as registered voters, and 1065 self-identify as likely voters. So roughly 92% of voting age adults claim to be registered, and 74% of voting age adults are reported as being likely voters.
The reality? Even in the wave election of 2008, only 69% of voting age adults cast ballots in Wisconsin. So there are 5% in this poll who claim to be likely voters who likely won't show up. And the number of people who are lying about even being registered voters is also high -- there are roughly 4.4 million adults in the state, but only about 3.5 million registered voters, or a little over 80%. So 10% of the people who claim to be registered in the poll aren't registered at all.
A lot of people who read media polls are going to be surprised next week. But I'll bet the Obama campaign won't be. They have internal polls showing they are actually losing Wisconsin, and that's why Obama is coming here twice this week.