Fifty-four delegates are up for grabs in the latest contest in the race to become the GOP nominee for president, CNN reported.
"Romney could really use a romp in Illinois. It wouldn't put him over the top, but it would put him back on track with a head of steam," CNN Chief Political Correspondent Candy Crowley said.
After finishing third last Tuesday in Alabama and Mississippi to Santorum and former House Speaker Newt Gingrich, Romney regained a bit of momentum on Sunday when he trounced Santorum in Puerto Rico's primary, according to CNN.
Polls in Illinois show a widening lead for Romney. A Public Policy Poll taken over the weekend showed Romney leading Santorum, 45 to 30 percent. However, a Chicago Tribune/WGN poll taken 10 days before showed only a 4 point gap, the Christian Science Monitor reported.
Romney and Santorum spent much of Sunday campaigning across the state, laying the groundwork for a furious final push for votes heading into Tuesday's primary, the Chicago Tribune reported.
Romney's confidence was on display Monday, while Santorum was forced on the defensive for first declaring that the economy was not a top issue in the campaign and then stating that "the campaign doesn't hinge on unemployment rates," The Associated Press reported.
Romney planned to hold an election-night party Tuesday in Schaumburg, Ill.
Santorum was leaving the state to deliver an evening speech about Abraham Lincoln in Gettysburg, Pa., according to the AP.