Observers see Illinois as a key race as Mitt Romney, Rick Santorum, Newt Gingrich and Ron Paul march onward to the Republican National Convention in Tampa, Fla., in August.
Romney, the former Massachusetts governor, and Santorum, the former U.S. senator from Pennsylvania, both have something to prove in Illinois.
For Romney, it's whether his uber-organization and monetary advantages will be enough to give him a win, maybe even an outright majority.
"Romney needs to spend a lot of time and money in Illinois in order to ensure a victory there," political commentator Steven Schier, a political science professor at Carleton College in Northfield, Minn., said. "He particularly needs to work the suburban areas around Chicago, where his support is greatest, and produce a large turnout there."
Santorum, meanwhile, is faced with trying to win a state that isn't as dominated by conservatives and evangelicals as those where his previous wins have come. Santorum's campaign also failed to file full delegate slates in four congressional districts, which skew the delegate count away from him no matter what happens in the beauty contest.
Gingrich took his second-place finishes in Mississippi and Alabama last week and headed to Illinois to campaign, repeating his vow not to bow out of the race. Paul, too, was on the stump.
"For the first time in decades, Illinois will be a factor in the presidential primary," Jim Nowlan, a political scientist at the University of Illinois' Institute for Government and Public Affairs, told the Rockford Register-Star.
"It's exciting," state GOP Chairman Pat Brady told the Chicago Tribune. "Illinois will be important. We're the only game in town that day."
A Chicago Tribune poll released early last week indicated Romney outpolled Santorum, 35 percent to 31 percent. Former House Speaker Gingrich and U.S. Representative Paul of Texas were favored by 16 percent and 7 percent of those polled.
However, the Tribune poll also demonstrated the changeability of the race: 46 percent of likely voters said they could change their minds before they vote Tuesday. (The survey of 600 Republicans registered in Illinois had a margin of error of 4 percentage points.)
On Friday, a Rasmussen Reports poll indicated Romney held a 41 percent-to-32 percent margin over Santorum, with a margin of error of 4 percentage points.
In the delegate snafu, Santorum named just 44 of 54 delegates in the state while Romney, Gingrich and Paul have full complements.
Illinois voters pick individual delegates -- 54 in all -- by congressional district. Though each delegate ostensibly is pledged to a candidate, they don't actually get pledged until the state convention where an additional 15 unbound delegates are assigned.
Even if Santorum wins the popular vote in Illinois, he can win only 44 delegates because he didn't nominate delegates in all the districts as the other major candidates did.
For Romney, Illinois is similar to Ohio, where he narrowly beat Santorum on Super Tuesday. Romney won the Buckeye State with large margins of victory in urban-suburban precincts that overshadowed Santorum's wins in more conservative, rural precincts.
The Illinois primary is an open primary, meaning anyone of any political stripe can come to the party as long as they're registered -- which is a wild card factor, The Hill said.
In the other open primary states of Michigan and Ohio, Democrats were encouraged to go to the polls and vote for Santorum, making the races much more competitive.
"A victory in Illinois by whatever margin is probably adequate to ensure Romney's momentum toward the nomination," Schier said. "He has over 50 percent of the delegates already chosen [in primaries and caucuses] and in April, winner-take-all states in the Northeast and West come into play. So far, those are his strongest regions."
The Illinois party primaries have some interesting down-ticket races as well.
On the Republican ticket, a confrontation pits Reps. Adam Kinzinger and Don Manzullo under remapped congressional districts, creating a schism between the House of Representatives GOP leadership and Tea Party advocates to boot.
Republican leaders, starting with House Speaker John Boehner have either endorsed or contributed to Kinzinger, considered a rising star in the party, The Hill said. However, Tea Party activists who supported Kinzinger's candidacy in 2010 say they feel let down by his more centrist voting record and are backing Manzullo.
Manzullo told The Hill he's "not surprised" that House leaders are backing Kinzinger, saying, "The leadership thinks he's someone they can work with."
Kinzinger has a different take.
"This race isn't about who's more conservative -- we're both conservative, everyone recognizes that," he told The Hill. "The people who have worked with him and worked with me are saying for the future of this party and the future of the country I'm the right choice … Don's a nice guy but he's been there 20 years. It's time to turn the page and get a new generation of leaders."
On the Democratic side, Jesse Jackson Jr., the only incumbent involved in a hotly contested primary against Debbie Halverson, who is trying to return to the House, Roll Call reported. Jackson's campaign released a poll he commissioned that indicated he had a seemingly insurmountable, 36 percentage point lead on Halverson. Initially, Jackson looked vulnerable because of his involvement in former Gov. Rod Blagojevich's pay-to-play scandal involving an appointment to fill President Obama's former Senate seat.