If the Public Policy Polling survey is accurate, and Rick Santorum really is 15 points ahead of Mitt Romney, we’re about to witness another sea change in this most tumultuous of GOP presidential primaries.

I had wondered if a big win in Arizona might be a higher priority for Santorum; if Newt Gingrich failed to get traction in Arizona, one could reasonably ask where he could win, outside of South Carolina so far (and his home state of Georgia). But Gingrich may already be an afterthought (11 percent in Michigan, 17 percent in California, 16-17 percent nationally, 13 percent in Pennsylvania). Santorum has already established himself as the top rival to Romney; now he’s competing with him for frontrunner status.

Beating Romney in Michigan… would be a game-changer. Mitt Romney won four years ago by a wide margin when his campaign needed it most. Romney began his career there; his father, George Romney, was a two-term governor and successful auto executive there. (Of course, this factor is easily overstated; roughly 65 percent of Michiganders were not alive when Romney was last governor, in 1969. What’s more, Michigan’s population was estimated at 10,002,486 in 2008 and 9,876,187 in 2011, meaning that while the Michigan exodus has slowed in recent years, some of Romney’s backers from four years ago may not be around this cycle. The PPP survey notes that only 26 percent of primary voters think of Romney as a Michigander (interestingly, 33 percent of Romney backers don’t consider him to be one).

Four years ago, Romney swept the more urban eastern counties, while McCain carried most of the western counties and the upper peninsula. Romney ran up his margin in the counties with highest raw turnout, 25,326 Romney votes in Kent (to McCain’s 20,927),  35,022 in Macomb (to McCain’s 19,042) 43,853 in Wayne (to McCain’s 25,778) and 62,298 in Oakland (to McCain’s 35,752).