Republican presidential candidate, and former senator Rick Santorum and his wife Karen check out a foam cheese hat during a campaign stop at Simon's Specialty Cheese in Appleton, Wis., on Monday. (Scott Olson/Getty Images)

For the third time in a month, the Great Lakes region plays host to the marquee contest in the GOP presidential race. And former Pennsylvania senator Rick Santorum, once again, is looking for the upset.

He’s unlikely to get it, but polling suggests that former Massachusetts governor Mitt Romney won’t win in a blowout so there’s enough doubt to make things interesting.

Which is where we come in. Below, we’ve listed five counties to watch as the results start rolling in. What happens in these five counties will tell you about all you need to know about which direction the race is heading, and whether Romney can pull what would appear to be a three-state/District sweep tonight. (The other contests are in Maryland and the District of Columbia where Romney is a heavy favorite).

Below our five counties to watch is a scorecard of the 2008 results, complete with targets that Romney needs to hit tonight to win.

* Dane: There hasn’t been much talk about an “Operation Hilarity” in Wisconsin – i.e. Democrats crossing over to vote for Santorum in hopes of prolonging the GOP race – but if it happens, it happens here. Dane County is home to the heavily liberal city of Madison and the University of Wisconsin. And even if Democrats don’t turn out, Dane is still a big population center; it comprised 6 percent of the primary vote in 2008, making it the third-biggest county in that race.

* Eau Claire: This county should give us a good gauge regarding Santorum’s strength in the more rural parts of the state. It was Arizona Sen. John McCain’s second-worst county in the 2008 primary and is full of socially conservative voters and evangelicals. Santorum needs to score a huge margin in Eau Claire and counties like it, taking at least two-thirds of the vote to cover his losses in more urban areas.

* Milwaukee: This is the state’s biggest and most urban county, and it should be Romney territory. Milwaukee comprised 12 percent of the primary vote in 2008 and gave McCain a 62 percent to 29 percent margin over former Arkansas governor Mike Huckabee. As long as Romney wins a strong majority here (we’re thinking mid-50s), he’ll be on a path for victory. (Interesting fact: Milwaukee is originally an Algonquin term meaning “the good land”. Thank you, Alice Cooper.)

* Racine: This has often been the state’s big bellwether county, both in the primary and the general election, as it exemplifies the kind of business-centric Republicans that dominate Wisconsin. Consider this: In 2008, McCain won the state’s primary 55 percent to 37 percent; he won Racine County 58 percent to 35 percent. It’s also the state’s fifth-biggest county.

* Waukesha: This is the second-biggest county in the GOP primary, comprising slightly less of the vote than Milwaukee County. It is highly suburban and very conservative, but it’s also filled with the kind of economic-oriented conservatives that Romney can and should win by a sizeable margin. If Romney wins this race, he wins it in Milwaukee, Racine and Waukesha.

Below, we have compiled the county-by-county results from 2008, along with an estimate (based on the 2008 results) of how Romney needs to do in each county to be on-path for victory.

(And click here for the Google spreadsheet.)