FACE OFF 
 
 Illinois is a key front in a GOP contest that has become political trench warfare. 
 
 The Illinois primary Tuesday is the next key marker in the grinding race for the Republican nomination, the latest front in a GOP contest that has become political trench warfare between Mitt Romney and Rick Santorum. Here are five things to watch as the returns come in: 
 
 DELEGATES VERSUS POPULAR VOTE 
 
 When Illinois Republicans step into voting booths tomorrow to cast ballots for a presidential candidate, those votes will be decide nothing, but merely register in a symbolic beauty contest, like the primary held in Missouri ahead of that state’s caucus. Illinois’ delegates, who voters choose by their name (not the name of the presidential candidate) in each Congressional District, will be elected further down the ballot. So if, for example, Santorum voters, (who tend to be less well educated than Romney voters, according to exit polls) neglect to check the down-ballot boxes in the delegate race, their candidate could win the popular vote in a congressional district but not pick up any delegates. 
 
 GINGRICH DELEGATES IN SANTORUM DISTRICT 
 
 In four congressional districts—the 4th, 5th, 7th and 13th—the Santorum campaign didn’t file the paperwork for delegates. Thus, while voters can vote for Santorum in those districts, they will only be able to vote for delegates pledged to Romney, Gingrich or Paul. This could be an opportunity for Gingrich to capitalize on the “anti-Romney” vote to rack up as many as 12 delegates. in a state where he’s not otherwise competitive. Since the key question in the race has become whether Romney can accumulate enough delegates to lock up the nomination before the convention, the Santorum campaign won’t complain if Gingrich pursues this strategy. 
 
 CHICAGOLAND VERSUS DOWNSTATE 
 
 Like the Korean Peninsula, Illinois is cleft into two parts: Chicagoland and “downstate.” Chicagoland is Cook County and the five “collar counties” that surround it. Downstate is everything else, from high bluffs facing Iowa on the Mississippi River, to the southern twang of “little Dixie” along the Ohio River and everything in between. So far, the Republican contests have registered an urban-rural split, with Romney consistently winning big cities and prosperous suburbs, while Rick Santorum has been the only candidate who can “play in Peoria.” Among the Illinois Republican primary electorate, these two areas vote in about equal proportions (although downstate is significantly less populated than metro Chicago, it is also significantly more Republican). The question is whether Santorum can do well enough downstate to balance out the margins that Romney will likely pile up on the shores of Lake Michigan. 
 
 REDISTRICTING 
 
 Illinois was one of the few states in the country where Democrats controlled the redistricting process this year. The result is a map that is carefully constructed to create as many Democratic seats as possible. Many districts are tadpole shaped, their heads inside Chicago city limits and long tails reaching out into the suburbs and beyond, entrapping as many Republican voters as possible, while maintaining a Democratic advantage. The result is that several congressional districts will have a majority of their population inside Chicago but a majority of their Republican primary voters will be out in the suburbs and exurbs. In contrast, while the base Democratic vote is widely dispersed, many Republican voters will be packed into a handful of congressional districts. This means the areas with the most Republican votes are usually not the most efficient places to campaign for delegates. The result will be that the GOP primary will be taking place on a peculiar battlefield, which does not follow the state’s traditional cultural or geographical divides. 
 
 MANZULLO V KINZINGER ||||| After three consecutive Tuesdays marked by multi-state primaries, the GOP presidential race returns to the single-state showdown: Illinois. 
 
 The contest, largely waged between Mitt Romney and Rick Santorum since Newt Gingrich and Ron Paul all but gave up on the state, represents the second-biggest delegate haul to date — 69 in total. 
 
 Text Size - 
 
 + 
 
 reset Driving the Day: Illinois votes 
 
 Below is POLITICO’s list of five things to watch: 
 
 Mitt’s score 
 
 Romney hasn’t won a lot of clean majorities. When he’s captured more than 50 percent of the vote, it has tended to be in caucus states, places outside the continental U.S. or in states with some caveat or another. 
 
 While it seems unlikely given recent polling, if Romney does manage to win more than 50 percent in this Midwestern mega-state, it will send a powerful signal that the clock is about to expire for his GOP rivals. 
 
 But even if he doesn’t hit that threshold, a double-digit win will continue to poke holes in Rick Santorum’s theory of the case. Romney will have won every industrial Midwestern state that has voted so far. And a big win will almost certainly be powered by a strong performance in the Chicagoland area, which will reinforce the notion that Santorum is unable to win in the suburbs — a constituency that will be critical in the fall. 
 
 The collar counties 
 
 If you’re looking for the Illinois equivalent of Romney country, it would likely be the group of counties outside the city of Chicago and its surrounding Cook County: DuPage, Kane, Lake, McHenry, and Will counties. They’re seemingly built for a candidate like Romney: They are Republican by heritage, more fiscally than socially conservative and more affluent than the rest of the state. For decades, they’ve voted in tandem with the suburbs and exurbs of other big cities, moving from rock-ribbed Republicanism to something closer to swing county status. 
 
 After voting for Barack Obama in 2008, all of them turned around and voted Republican by wide margins in the 2010 races for governor and Senate. 
 
 If Romney can’t run up the score here, he’s got serious problems in Illinois. But he’s carried the suburban vote in most states so far and there’s little reason to think he won’t do it again. One promising sign: an early March Chicago Tribune/WGN poll showed Romney with a comfortable 39 percent to 27 percent lead in the collar counties. 
 
 Santorum’s limits 
 
 The exit polls frame Santorum as a candidate whose support exists within very narrow boundaries. He runs well with evangelical voters, rural voters and the very conservative, but he’s struggled to break out beyond those groups. 
 
 Romney has his own limitations, of course, but he has still managed to win a broader range of GOP voters. And he has an ace in the hole: even in states he loses, the former Massachusetts governor is rated by voters as the candidate most likely to defeat Obama.
– Illinois votes for its GOP candidate of choice today, with 69 delegates at stake—the second-most of any state so far. Here's what the experts at Politico and the Daily Beast have their eye on: Can Mitt Romney crack 50%? Polls have Romney heavily favored in the state, and with Ron Paul and Newt Gingrich barely trying, passing the simple majority mark is possible. That would "send a powerful signal that the clock is about to expire for his GOP rivals," writes Charles Mahtesian for Politico. Delegates vs. votes: When voters check a candidate's name in this primary, they're just expressing a preference—their actual delegates are selected by name further down the ballot. Santorum's supporters are generally less-well-educated, and might neglect to mark both, Ben Jacobs muses at the Daily Beast, giving Romney delegates in counties Santorum "won." Santorum fans' second choice: Santorum didn't file paperwork to get delegates on the ballot in four districts, so people voting for him will have only Romney, Gingrich, and Paul delegates to pick from. Will they gravitate toward Gingrich as an anti-Romney vote? Can Romney win downstate? Illinois is split between urban "Chicagoland," (Cook County and the five affluent "collar counties" around it) and rural "downstate" (everywhere else). Chicagoland is perfect for Romney, while downstate looks like a Santorum state. Will Newt crack 10%? Polling has him at 13%, but among early voters that fell to 8%. A single-digit finish would, needless to say, be highly embarrassing.