Although it looked to me like the Wisconsin primary results confirmed an erosion in the Clinton coalition starting with the Potomac primaries, Jay Cost works his number-crunching magic and concludes that the erosion more likely starts in Wisconsin:

I think this shift is hard to account for without hypothesizing a momentum effect. Consider the following. We know that the Hispanic population, Catholic population, and union population are important factors in how each candidate performs. However, the difference in the Hispanic population between Virginia and Wisconsin is negligible (4.7% in Virginia to 3.6% in Wisconsin). There are big differences the differences in the Catholic and union populations, but these should favor Clinton. The explanatory variable that is not accounted for above that favors Obama is racial homogeneity. It is possible that lower income Wisconsin voters are more amenable to Obama because race is perceived differently in Wisconsin than it is in Virginia. Indeed, I think a strong argument can be made that this was indeed a factor. However, I do not think this could explain the whole of this shift.

All in all, the exit polls and the aggregate vote results seem to point in the same direction – a shift in Wisconsin, but not in Virginia and Maryland.

Not that this will make Sen. Hillary Clinton feel any better about the dire state of her campaign.  As usual, I recommend the whole thing, with the data plots and all that jazz.