Rick Santorum has slipped ahead of the Mitt Romney in the polls, marking an ignominious milestone in the Republican nomination: Since last summer, when Romney was at the top of the early polls, the lead has switched nine times. In order, it’s gone to Rick Perry, Romney, Herman Cain, Romney, Gingrich, Romney, Gingrich, Romney, and Santorum, who now leads the Real Clear Politics’ aggregated trend with 30.2 percent to Romney’s 28.6 percent. Notice a pattern?

The Signal continues to predict that Romney will win the nomination. According to the prediction markets, he has a 72.8 percent likelihood to win the nomination, followed by Santorum at a non-negligible 17.8 percent. That’s a far more vulnerable position for the former Massachusetts governor than he found himself in a few weeks ago, but it’s still an uphill battle for Santorum. On the following chart, the vertical line represents when the first polls closed on Tuesday, February 7, when Santorum won three primary states (two for delegates and one beauty contest).

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