CHICAGO (CBS) — Voters are heading to the polls Tuesday morning for an Illinois primary election that experts have said could be “defining” for the Republican presidential race.

In the Republican race, former Pennsylvania U.S. Sen. Rick Santorum is looking for an upset over former Massachusetts Gov. Mitt Romney. Polls opened at 6 a.m., on the heels of a last-minute war of words between the two candidates.

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Santorum says he is looking for a do-over after comments he made in Moline Monday about the economy.

“We need a candidate who’s going to be a fighter for freedom; who’s going to get up and make that the central theme in this race, because it is the central theme in this race,” Santorum said at the Monday event. “I don’t care what the unemployment rate’s going to be; doesn’t matter to me.”

At a later event in Peoria, Romney was quick to take issue with Santorum’s remarks.

“One of the people who’s running also for the Republican nomination today said that he doesn’t care about the unemployment rate; that doesn’t bother him,” Romney said “I do care about the unemployment rate. It does bother me. I want to put people back to work.”

Meanwhile, Santorum tried to explain what he meant.

“Of course I care about the unemployment rate. I want the unemployment rate to go down,” Santorum said. “But I’m saying my candidacy doesn’t hinge on whether the unemployment rate goes up and down. Our candidacy’s about something that transcends that – it’s about freedom.”

CBS News’ Susan McGinnis reports Romney has an advantage even before the polls open.

Santorum didn’t file necessary paperwork in four Illinois districts, making him ineligible for 10 of Illinois’ 54 delegates at stake Tuesday.

That could help Romney extend his lead as he tries to clinch the nomination before the convention in August.

In an interview on the CBS 2 News at 11 a.m. Monday, U.S. Sen. and former presidential candidate John McCain (R-Ariz.), a Romney supporter, said the Illinois primary may be the “defining” contest in the race.

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“Perhaps the defining primary is now in the heartland of America – in the state of Illinois – a diverse state, and I think that when, and if, Mitt Romney wins tomorrow, it’ll give an inevitability to the campaign that is much needed,” McCain said.

Meanwhile, numerous other key races are up for grabs on both the Democratic and Republican side.

• U.S. Rep. Jesse Jackson Jr. (D-Ill.) is facing his strongest challenge in years from fellow Democrat Debbie Halvorson, who lost her 11th District seat to Republican Adam Kinzinger two years ago. Jackson’s 2nd District has since been redrawn to include parts of Halvorson’s old district.

• In the 8th Congressional District in the north and northwest suburbs, Iraq War veteran and former Illinois Department of Veterans Affairs Director Tammy Duckworth is up against former Deputy Illinois Treasurer Raja Krishnamoorthi. The winner will take on Tea Party firebrand Rep. Joe Walsh (R-Ill.) in the general election.

• In the 16th Congressional District in north central Illinois, a Democrat-drawn remap has pitted the freshman Kinzinger (R-Ill.) an Air National Guard pilot, against Rep. Donald Manzullo (R-Ill.), who has been in Congress since 1992. Both are conservative, but the Tea Party has endorsed Manzullo.

LISTEN: WBBM Newsradio Political Editor Craig Dellimore reports

• In Cook County, the most competitive race is for Cook County Clerk of the Circuit Court. Ald. Ricardo Munoz (22nd) is taking on three-term incumbent Dorothy Brown. Both candidates are focusing on the need to modernize the county’s court recordkeeping system.

LISTEN: WBBM Newsradio Political Editor Craig Dellimore reports

• In Kane County, the Republican race for County Board Chairman has been a heated one, pitting state Sen. Chris Lauzen (R-Aurora) against Geneva Mayor Kevin Burns. On the Democratic side, former St. Charles Mayor Sue Klinkahamer is facing Carpentersville Mayor Bill Sarto.

• With Lake County State’s Attorney Michael Waller stepping down, there are full fields of candidates on both sides.

• To the south, Will County Executive Larry Walsh is facing a challenge from fellow Democrat Leonard Thompson. Will County Forest Preserve Board President Cory Singer is running unopposed for the office on the Republican side.

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• In a tight race for the Illinois Supreme Court, appointed Justice Mary Jane Theis is up against Appellate Court justices Aurelia Pucinski and Joy Virginia Cunningham, and attorney Thomas Flannigan.