Barack Obama leads the top Republican candidates for President in Wisconsin, but for the most part his margins over them are a good deal less than what he won against John McCain in 2008- this is one state where Obama's position clearly has weakened over the last couple years.
Mitt Romney comes closest to Obama, trailing 46-42. Obama's lead over Mike Huckabee is 47-41 and against Newt Gingrich it's 50-41. Only against Sarah Palin does Obama match his 14 point margin from 2008- he leads her 52-38.
The big differences for Obama between now and 2008 come with Republicans and independents. Against McCain exit polls showed Obama taking 10% of the Republican vote. That may not sound like much- and it isn't- but that's a strong performance compared to the 2% against Romney, 4% against Huckabee, 5% against Gingrich, and 6% against Palin that Obama gets now. Whatever little crossover support he once had has pretty much evaporated.
With independents Obama actually does lead all of the Republicans- by 7 points over Romney, 9 over Huckabee, 16 against Gingrich, and 20% against Palin. But again- with the exception of the Palin match up- those margins don't hold up to the 19 point win with independents exit polls showed him earning against McCain last time around.
Opinions about Obama in the state are divided almost evenly with 47% of voters approving of him and 46% disapproving. You might expect his horse race numbers to be even weaker with those kinds of approval figures but as is the case elsewhere voters in Wisconsin look very unfavorably upon the Republican field. Mitt Romney is the most 'popular,' if you can call it that, with 33% of voters holding a favorable opinion of him to 45% with a negative one. Mike Huckabee's reviews are pretty similar at 32% favorable and an identical 45% unfavorable. The other two Republicans are far more unpopular with Sarah Palin's at a 35/58 spread and Newt Gingrich at 28/54. Obama's best ally as he heads toward reelection is the likely Republican candidate field.
Obama would win Wisconsin again if the election was today- and he'll probably win it in 2012 if the GOP nominee is one of these folks- but his numbers in Wisconsin still speak to a weakened position for him in the Midwest relative to 2008.
Full results here