Former Massachusetts governor Mitt Romney is expected to cruise to victory over Pennsylvania senator Rick Santorum in tonight’s Illinois Republican presidential primary.

A Voter enters the fire station polling place March 20, 2012 in Magnolia, Illinois. (DON EMMERT/AFP/Getty Images)

Below are five counties to watch in tonight’s vote — gleaned from conversations with smart Illinois Republicans and a bit of our own insight. (The counties are listed alphabetically.)

* Cook County:Cook County contains Chicago — not exactly a Republican bastion — but also has suburban areas within it where fiscally conservative, socially moderate Republicans live. Given that demographic makeup and the fact that Romney spent heavily in the Chicago media market, suburban Cook should go strongly for him tonight.

* DuPage County:One of the so-called “collar counties”ringing Chicago, DuPage sits directly west of Cook County and includes the city of Naperville. Once staunchly conservative, DuPage has grown more and more centrist as people migrate out of Chicago/Cook County and into DuPage. In 2004, President George W. Bush carried DuPage with 54 percent; four years later, President Obama won it with 55 percent. Again, this should be a Romney County.

* Kane County:West of DuPage, Kane is the fastest growing collar county — it’s population increased by 27.5 percent between 2000 and 2010, according to the Census Bureau. Kane was once a basically rural county outside of Aurora and Elgin — its two big cities — but the rapid growth has turned it into a suburban/exurban county. Obama carried Kane with 55 percent in 2008 but this should be strong Romney country tonight. (Fun fact: The Fix in-laws reside — for part of the year — in Kane County.)

* Peoria County: This is middle America. (It’s where the famous phrase “Will it play in Peoria” — meant to signify whether something could appeal to mainstream America — originates.) This central Illinois district has a strong tea party contingent and is dominated by agricultural interests. (It’s also the home of Caterpillar.) Peoria should be a good Santorum county.

* Winnebago County: Winnebago includes the city of Rockford, the third largest city in the state. It’s an interesting mix of urban, suburban and rural voters. Given that cross section, Winnebago is a toss-up county between Romney, who runs better in urban and suburban areas, and Santorum, who runs better in rural areas. Winnebago is also ground zero in the very competitive Republican primary race between Reps. Don Manzullo and Adam Kinzinger . One complicating factor: There are reports that ballots in the county are too large to fit into the scanning machines, meaning that they will have to be counted by hand.