Rick Santorum won two surprise victories last night in the Alabama and Mississippi primaries, and he did so by poaching voters from Newt Gingrich’s coalition. To appreciate this, let’s take a look at some data.

First, the topline numbers in the four Deep Southern states that have voted so far.

As we can see, Romney won roughly the same share of the vote in all four states – something within a very narrow band of 26 percent and 30 percent of the vote. The real shift last night came from Santorum surging at the expense of Newt Gingrich.

We can also see this when we break it down into ideological groupings, above all “very conservative” voters. This, from the exit polls:

Here we see a similar pattern – Mitt Romney holding in a very narrow band (between 18 percent and 24 percent) while Rick Santorum has surged by about 15 points at the expense of Newt Gingrich, who has declined by 15 points.

Importantly, something similar happened with the rural vote, which was Gingrich’s bread-and-butter in Georgia and South Carolina. Last night we saw a different result.

Again, we see a real collapse in Gingrich’s support – the difference this time is that it looks as though Mitt Romney was the beneficiary at least to some extent.

From a bigger picture, these results confirm what the RealClearPolitics average has been saying for about a month: Rick Santorum has eclipsed Newt Gingrich in the battle to be the conservative alternative to Mitt Romney.

Romney’s line is in purple, Santorum’s line is in brown, and Gingrich’s is in green. Gingrich and Santorum crossed paths around February 9th in the nationwide polls; that Gingrich could not even eke out wins in his home region tonight signals that his candidacy stands virtually no chance of outright victory, although it is unlikely he will bow out of the race.

And what about the race to win the 1,114 delegates needed to capture the nomination? Sean Trende wrote an important column yesterday observing that no candidates seem to have generated much “momentum,” such that wins in yesterday’s contests propel them forward for tomorrow’s battle. Instead, Trende sees (accurately, in my view) a slog through demographic encampments, with nobody poised to cut into the opposition’s strongholds any time soon.

That was confirmed tonight. Romney basically did as well in Alabama and Mississippi as he did in Georgia and South Carolina, and the conservative Santorum’s victory came at the expense of the conservative Gingrich’s core voters.

The problem for Santorum is that he only netted six delegates over Romney in these two contests – and I am guessing that as of Wednesday morning (after American Samoa and Hawaii come in) Romney will have won the most delegates for the whole day. So, with many of Santorum’s best states now having finished voting – the caucuses in the Farm Belt and now much of the South – Romney still has a large delegate lead, with both California and New York still to come.

Thus, last night confirmed for me what I think is the dominant theme of this race: Romney is far from the consensus choice of the Grand Old Party, but his position in this race is strong enough that it is difficult to see how Santorum takes the nomination from him without changing pretty dramatically the dynamics of the race.

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