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).

Sources: Betfair and Intrade

Santorum has also but he has overtaken Romney in recent polls in Michigan. On the strength of recent polls by American Research Group and PPP--two polling organizations which use questionable methodologies--the New York Time's Nate Silver has Santorum 75 percent likely to carry the state where Romney was born, his father served as governor, and he carried easily in 2008.

We are a little more cautious with Santorum; The markets suggest he is 55.7 percent likely to carry Michigan to Romney's 44.7 percent. These numbers have certainly moved in Santorum's favor over the last few days, but not nearly as much as in those models that rely completely on polling.

The reason that our numbers differ from the polling numbers is because we are looking forward, where the polls attempt to take a snapshot of the world today. Romney has serious advantages over Santorum as the Michigan primary and the rest of the GOP nominating contests progress. Romney has much more money and organization than Santorum. And the polls so far have reliably returned to Romney after every flirtation (or re-flirtation) with another candidate, with the regularity of a dippy bird.

Story continues

Follow along on PredictWise for the real-time likelihood of the upcoming republican primaries, the Republican nomination, and the presidential election.

David Rothschild is an economist at Yahoo! Research. He has a Ph.D. in applied economics from the Wharton School of Business at the University of Pennsylvania. His dissertation is in creating aggregated forecasts from individual-level information. Follow him on Twitter @DavMicRot and email him at thesignal@yahoo-inc.com.

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