In earlier states, we've seen that Obama tends to fare well among independent voters. Wisconsin has an open primary, which means that Obama can again benefit from independents voting in the Democratic primary. However, trying to figure out exactly what independent turnout will be, and which primaries those independents will vote in is a bit trickier.
In 2004, exit polls indicated that independents made up 29% of those voting in the Democratic primary. Republicans made up 9% of the Democratic electorate that year as well. Of course, the Republican primary that year was meaningless since Bush was running unopposed. This year, some independents may choose to vote in the Republican primary, even if that party's nomination race is all but over. Thus, even if you factor in larger than normal turnout for the primary, it seems unlikely that more than 1/3 of the Democratic primary electorate will be comprised of independents or that more than 10% would be Republicans.
Public Policy Polling, a Democratic survey firm, is showing that Obama holds a marginal lead among Democrats, but a much stronger advantage among independents and Republicans. Overall, their poll shows a 53-40% lead for Obama. Their sample was comprised of 71% Democrats, 9% Republicans, and 20% independents. But what happens if you shift the numbers to make this year's electorate look like the one that turned out in 2004? If that happens, you have a larger share of independents (29% instead of 20%) and a smaller share of Democrats (62% instead of 71%). If you shift the numbers like that, you get Obama receiving 54%, Clinton with 38%, and 8% undecided. The gap between Obama and Clinton grows from 13% to 16%...not a huge difference, but it could be big enough to affect perceptions as well as an extra delegate or two.
But what does it mean for McCain if all the independents vote in the Democratic contest? Based on how the vote has divided in the earlier contests, we might expect it to hurt McCain, but the same survey suggest otherwise. In that poll, McCain actually held a 14% lead over Huckabee among Republicans but just a 4% lead with independents. However, American Research Group finds the opposite (and more expected pattern) in their survey. But McCain lead Huckabee among both groups, so he should win Wisconsin regardless.