Hillary Clinton suffered a stinging blow last night, losing Wisconsin by 17 points. What is most worrisome for her is that Obama seems to have broken into several of her core voting groups. This is the first real evidence of momentum we have seen on the Democratic side.

After the Potomac Primary last week, some argued that Obama had already begun to build momentum because of his large victories in Maryland, Virginia, and the District of Columbia. I thought this talk was hasty. Given the large number of African American voters in each contest, and given that white voters in all three primaries were quite wealthy - Obama's sizeable victories did not come as a surprise. In particular, 37% of Maryland voters and 30% of Virginia voters were African American; 39% of Maryland voters and 39% of Virginia voters claimed to make $100,000 or more per year. So, it is hard to argue that Obama's success was due to him peeling off portions of the Clinton coalition. What seems more likely is that he won handily because his best voting blocs were in good supply that day.

The same cannot be said for Wisconsin. Just 20% of Wisconsin Democratic voters claimed to make $100,000 or more per year. Only 8% of the electorate was African American. So, Wisconsin did not favor his strongest groups so heavily. Nevertheless, he still had an outstanding day. Consider the following chart, which uses the exit polls to compare Obama's margin of victory with key groups in the non-southern states to his performance with those same groups in Wisconsin last night.

So, for instance, Obama won white males in the non-South by 8 points. Last night, he won them by 26 points, yielding a net increase of 18 points.

As you can see, Obama enjoyed significant expansions in three of his four strongest demographic groups. Though it appears he did worse among white Protestants - the difference between the two is possibly due to statistical sampling error. So, all in all, Obama did not do significantly worse with any of his groups - and with most of them he did significantly better.

Meanwhile, he was able to peel away portions of Clinton's core electorate. To appreciate this, consider the following chart, which compares Clinton's margin of victory over Obama in the non-South among the groups that have typically favored her.

These numbers tell the tale succinctly. Clinton suffered significant losses across many of her core constituencies. White women, Democrats, union workers, downscale voters, and white Catholics all drifted to Obama last night - some so much that Obama actually won them.

Let's cross-reference the exit polls with a look at the actual vote returns. The following chart reviews Wisconsin counties according to median white income. Specifically, it divides 71 of Wisconsin's 72 counties into three tiers according to wealthiest, poorest, and middle. It then looks at Obama's performance in each tier - calculating his average share of the vote across the counties in the tier, and then his share of the vote for the whole tier. [Note that no returns were available for Clark County by the time I had to put this chart together. Most counties had complete returns, and the ones with partial returns were all mostly reported. So, these numbers will probably be different when all the returns are in, but only slightly so.]

As you can see, Obama did better as whites get wealthier. However, he does very well across all tiers. This confirms what the exit polls indicate: Obama retained his own strengths while cutting into Clinton's strengths. We would have expected her to win the poorer counties, but in fact she lost them.

I was also interested in looking at vote returns according to population density. If Clinton was doing well with economically "down scale" voters - it stands to reason that her strength in a state like Wisconsin would be with counties that are sparsely population. Obama, on the other hand, would do well with counties with dense population centers. This dichotomy probably would not work in a state with large minority populations because of Clinton's strength with Hispanics. However, in a state like Wisconsin, we might expect Clinton to do better in rural areas and Obama to do better in urban ones.

I tested this hypothesis by again dividing counties into three tiers, this time according to population density. The following chart reviews the results.

Again, we see a similar phenomenon. While Obama did best in the dense counties like Dane and Milwaukee, he also won a majority in the middle-density counties and the sparse counties. There seems to have been no urban-rural divide. The state went to Obama regardless of population density. This is another indication that Obama cut into strengths that Clinton has enjoyed in previous states.

Is this a momentum effect? The word "momentum" has been tossed around way too much this cycle, which is funny because prior to tonight there has really been little evidence of momentum at all! So, what of last night? Are these bona fide expansions in his voting coalition, or was Wisconsin following the same pattern that the previous states have followed? It is impossible to be sure. On the one hand, I have found that Obama does well with whites in states where there are few African Americans. Duke's Brendan Nyhan has found the same trend. White voters in homogeneously white states seem to be more amenable to him than whites in diverse states. Wisconsin is a state with few African Americans. This probably gave Obama a boost last night. If this was a major factor - perhaps some of this apparent momentum effect would "wash out." On the other hand, could the racial homogeneity of Wisconsin alone really account for these huge shifts? That's a tough pill to swallow.

Unfortunately, we simply do not yet have enough data to give a definitive answer. Ideally, we could answer the question if we had enough observations to make a prediction of how Obama should have done in Wisconsin, given the results in past states. We could then compare the actual results to the prediction and see whether the difference between the two is statistically significant. That difference, if it is significant, could be a momentum effect.

My sense is that a momentum effect of indeterminable magnitude emerged last night. That is, Obama's victories - most recently last week in the Potomac Primary - contributed to the size of last night's victory, though he would have done very well if there had been no momentum. This is the first Democratic contest where I think a case like this can be made. Until last week the typical ebb-and-flow of each candidate's demographic strengths could probably account for the actual results.

This surprises me a little bit. As I noted last week - it is rare for a front running candidate like Obama to develop momentum like this. It is not unheard of (e.g. George H.W. Bush had something like this in 1988), but it is rare. Usually, candidates like Obama are "launched" with a splashy win in New Hampshire. That is different from what seems to be happening here. Obama seems to have built upon his latest wins.

If there is a sizeable momentum effect, Clinton should be very nervous. Demographically, Texas has a lot in common with California, except that there are more African Americans. This bodes well for Obama - and if momentum is now in the equation, Clinton could be in real trouble. If you take the margin of her California victory, factor in the larger African American base, and factor in a 5 to 7 point shift in the white vote to Obama - that win might become a loss.