The map on this page shows not only who’s winning in each of the state’s 102 counties, but, using Patchwork Nation, it shows how the candidates are doing in each of Illinois’ 10 types of county – from the wealthy Monied Burbs to the rural agricultural Tractor Country counties. It will fill in with data as the results from the primary comes in.

What to watch for:

Illinois is indeed a big complicated state, home to some 13 million people and 10 of Patchwork Nation's 12 types. But the overwhelming majority of the state's population -- more than 8 million -- lives in just two of those types: the big city Industrial Metropolis counties and the wealthy Monied Burbs.

Those types of counties have been good to Romney in the 2012 Republican primaries and caucuses. He's won 43 percent of the vote coming out of each of those types in contests so far. There are also another million voters in Illinois collegiate Campus and Careers counties, in dark green on the map above. Those places have also been good to Romney in this primary cycle.

There have been some signs that Santorum may be starting to pick up some strength in the Burbs. He actually won those counties in Alabama and Mississippi last week (states where the vote tends to be more evangelical in character across the board), and he challenged Romney in Ohio's Burbs, losing those counties by only 3 percentage points.

Even if Santorum gets a nice push in the Burbs and draws Ohio-like numbers there, Romney has done much better in the big city Industrial Metros -- in Ohio and nearly everywhere else. Santorum's focus on social issues is likely hurting him in those large cosmopolitan counties.

If elections were decided on acreage rather than votes, Santorum might feel better about Illinois. One of his biggest strengths has come in the small town Service Worker Centers, where his appeal to lower-income, blue-collar workers has struck a chord. But all those red counties only hold about 14 percent of Illinois's population. The best hope for Santorum in Illinois would be big turnout in those Service Worker counties, a few upsets in the Burbs and very low turnout in the Industrial Metros.

Could that happen? Of course. It has been a year full of surprises, and turnout can be hard to project in a primary. But barring something unusual, a big win in Illinois for Romney seems much more likely.