The persistence of these patterns leaves Romney strong enough to remain the clear favorite for the nomination, but probably not strong enough to drive Santorum from the race.
The race's most consistent dividing lines have been ideology, income, education, and especially religious affiliation. So far, exit polls have been conducted in 17 primaries and caucuses. Romney has won voters who describe themselves as moderate in 14 of them and tied Ron Paul among them in another. Voters who identify as somewhat conservative have gone for Romney in all but four states.
The former Massachusetts governor has carried voters earning at least $100,000 annually in every state except South Carolina and Georgia, where they broke for regional favorite Newt Gingrich. Those with at least a four-year college degree have backed Romney in all but five Southern and border states (including one where he tied Santorum among them).
Romney's most consistent and reliable support has come among voters who do not identify as evangelical Christians. He has carried those voters in every state with an exit poll except Gingrich's home turf of Georgia, and he routed Santorum among them in Illinois by 2-to-1.
With all of these more affluent, secular, and moderate groups in the party's managerial wing, Romney amassed even bigger advantages in Illinois than he did in Michigan and Ohio. That might be a one-state blip, but it could also signal that Santorum's hard-edged message, especially on social issues, is narrowing his appeal as the race proceeds.
Santorum's foundation is the mirror image of Romney's groups, particularly evangelical Christians, who flocked to him after the former senator from Pennsylvania revived his campaign with his three-state sweep on Feb. 7. The key limit constraining Santorum is that he still has not carried more than 31 percent of voters who do not identify as evangelicals in any state.
As long as these patterns hold, the GOP race will produce very few additional surprises. Romney will maintain a clear edge in states, mostly along the coasts, where evangelicals will likely comprise about only a third of the vote or less (and affluent, better-educated voters also loom larger), including Maryland, Connecticut, Delaware, New York, California, and New Jersey. Although Romney cut Santorum's winning margin among evangelicals in Illinois, the former senator remains the favorite in states where those voters will likely cast at least half of the GOP ballots, including in Louisiana, Texas, West Virginia, Arkansas, Kentucky, and probably North Carolina and Indiana (where no recent exit polls are available).
Still, if Romney can match his pattern in Illinois — and win non-evangelicals by a much larger margin than Santorum captures evangelicals — he might be able to surprise the former senator in states divided fairly closely between the two groups, such as Oregon, Nebraska, and maybe North Carolina. For all the focus on Romney's troubles with the most ardent conservatives, Santorum's inability to reach beyond them looks like an even more disabling problem after he lost resoundingly in another Midwestern showdown.