Ohio and Michigan are the closest facsimiles to Illinois, two Midwest primaries that featured narrow Romney wins. Each state has the same rough demographic sketch, consisting of a sizable population of college-educated, secular Republicans. White-collar Republicans constituted 51 percent of the vote in Michigan, for example - four years ago, the Illinois primary featured 52 percent of college-educated voters.
In Ohio, 47 percent of voters were evangelical, according to exit polls taken after this year's primary. That's a similar share to the 41 percent in Illinois in 2008. Both states are far less religious than other GOP primary states - three-quarters of the GOP vote in Alabama, for instance, was evangelical.
Despite the demographic edge, Romney won Michigan and Ohio by three and one point, respectively -- minute victories that would seem to portend another close contest in Illinois. But the circumstances this go-round are different. The onetime Bay State governor entered the previous two battles trailing his rival, sometimes by double digits. In Illinois, he enters the race with the lead already in hand.
A Chicago Tribune/WGN-TV poll released Sunday reported Romney in the lead at 35 percent, a small edge over Santorum's 31 percent. Ex-House Speaker Newt Gingrich, at 12 percent, and Rep. Ron Paul, 7 percent, each trailed far behind. And that was before Romney's TV campaign began to pound away at Santorum.
Santorum himself on Friday acknowledged he's an underdog.
"I don't think there was a single poll that had us up in Mississippi, so I don't really worry too much about polls," he told reporters after a speech in Osage Beach, Mo. "Look, Illinois's a tough state, I understand. When you're getting outspent 10 to 1, it's hard. But we're gonna keep working, and do the best we can. And we're fighting money, we're fighting the organization, and we're fighting a divided conservative vote. That's a pretty hard hill, but we've been climbing it and doing well."
As the former senator referenced, he's being outspent heavily in Illinois, by some estimates as much as five-to-one. That includes a new hard-hitting released Friday that attacks Santorum as an "economic lightweight" and questions his ability to beat President Obama in the fall.
If Santorum has hope, it lies with Illinois's more rural, conservative voters in the state's southern half. They're the voters who helped conservative state senator Bill Brady win the party's GOP nomination for governor in 2010 over Chicago-area lawmaker Kirk Dillard -- though Brady could not overtake Democrat Pat Quinn in the general election.
"Romney will do better much better in the Chicago suburbs than Santorum, and Santorum will do better than Romney downstate," Redfield said.
That regional success can help Santorum win a portion of the state's 54 delegates up for grabs on Tuesday, who are allocated based on support in congressional districts. As the ex-U.S. senator hinges his strategy on denying Romney the 1,144 delegates necessary for the nomination, those small successes could be critical.
Just don't count on his rewriting the GOP primary's script.