This is an interesting question. It's actually a part of a larger pattern in surrounding states. I won't post the whole series of dates, but just compare 1976 to 2012. Notice how in each case, rural areas adjacent to SW Wisconsin - in SE Minnesota, NW Illinois and NE Iowa - have shifted from R to D while many other rural areas in these states have shifted in the opposite direction, from D to R.
Minnesota, 1976:
Minnesota, 2012:
Iowa, 1976:
Iowa, 2012:
Illinois, 1976:
Illinois, 2012:
I think that there are three things going on here.
First, aside from the Iron Range, the Democrats have lost a lot of their rural (or at least small-city) industrial working-class vote. This is important in southern Illinois (as well as the upper south generally), but also in much of northern Wisconsin which has been historically dependent on the forest industry.
Second, there used to be a kind of farm-populist vote in the flat, fertile plains of western Minnesota and Iowa. This has dwindled as farms have consolidated and become more corporate. Some religious conservatism may also be important to this trend. Although northern Wisconsin is mostly forested, there is an area of flat prairie in the western "bulge" between Eau Claire and the Twin Cities that appears also to have undergone this trend.
Third, the rural driftless area along the upper Mississippi below the Twin Cities has shifted in the opposite direction. This area is agricultural, without much forestry or other industry, but has a hilly terrain that keeps farms smaller and less tied to big agribusiness, and it's also an area where the strands of Protestantism that have remained very socially conservative are rare. (In these respects it has some rough similarities to rural New England, which is likewise a historically Republican region that has gone Democratic).
