Most of the attention will focus on the state of Ohio. It's a populous state, and it's an important swing state in general elections. More significantly, Ohio is a Midwestern state, located in the battleground region for the 2012 Republican primary. The fight between Romney and Santorum will be decided in large part by what happens in the Midwest. The South is infertile territory for Romney, while the Northeast and West are less receptive to Santorum. The Midwest has a natural pro-Santorum bias, but isn't anti-Romney like the South, so both candidates will be contesting states in the Midwest.
Santorum's objective is to crush Gingrich in the South and crush Romney in the Midwest, hoping that big enough margins of victory in those two regions will be enough to put him over the top. Romney's objective is to sweep the Northeast and West while taking enough victories and delegates in the Midwest to accumulate a delegate majority. (Gingrich and Ron Paul currently have no path to victory.)
Arguably, Romney has the easier route: Santorum will be contested in the South by Gingrich and in the Midwest by Romney. Romney won't have as much competition in the Northeast or West. Those regions are also overrepresented in the "winner-take-all" portion of the primary season. In the Midwest, Santorum needs big wins, while Romney can afford some losses there. For Santorum, that's the bad news about having to fight on one's own turf. The good news is that a lot of the news coverage will be focused there, in a region where Santorum is on favorable ground. Meanwhile, Santorum really needs to hope that Gingrich drops out or evaporates in the South, because he can't afford to fight in two regions simultaneously.
Regardless of who wins Michigan, Ohio will be viewed as a rematch. Ohio should begin with a substantial bias in Santorum's favor--his home state of Pennsylvania is next door. Michigan was more of a neutral battleground because Romney could counter the regional bias with his own home state roots there. Polling currently suggests good news for Romney in Michigan, but even if he wins, he'll be starting out behind in Ohio.
While Ohio may have a natural affinity for Santorum, time is against him. The height of the Santorum surge--the "honeymoon" period during which voters assumed Santorum had no weaknesses--is past. Santorum has already lost quite a bit of ground since mid-February, but voters are still only just beginning to learn about Santorum's vulnerabilities. The Republican debate on February 22nd was a preview of the attacks against Santorum's conservative record. As we explained in our discussion of electability and conservatism, attacks against Santorum's electability will be more potent than those against his conservatism. Electability attacks have not yet begun.
Romney's campaign has chosen not to focus on electability, instead undermining Santorum's conservative credentials. If Santorum wins Michigan, or if it's too close for comfort, that is likely to change. Romney would become desperate, and things will get uglier. However, the media environment will become more toxic for Santorum over time, regardless of how well he does in Michigan. The same phenomenon occurred after Gingrich won South Carolina; liberal media and the Republican establishment united to make devastating attacks against Gingrich from which he never recovered. Santorum can expect to face a similar challenge leading up to Ohio and beyond.
