The nuts and bolts of the Illinois primary

(CNN) - Voters make their voices heard in the Illinois primary on Tuesday, casting their vote in the ongoing GOP presidential nominating contest.

With 54 delegates at stake, the state has already proved a prime battleground for Mitt Romney and Rick Santorum whose campaigns and supporting super PACs have spent millions of dollars in television ads attacking each other.

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As with most other states, Illinois allocates its delegates proportionally. Voters directly elect the 54 delegates in the state's 18 Congressional Districts.

Additionally, there are 12 statewide delegates reserved for a non-binding "beauty contest," which has no impact on delegate selection Tuesday and will later be selected at the state convention in June.

The total delegate count also includes three delegates for Republican National Committee members, which are not tied to Tuesday's primary results.

Since district delegates are directly chosen by voters, candidates must field a total of 54 delegates in the 18 districts in order to remain eligible for every delegate.

Former Massachusetts Gov. Romney, former House Speaker Newt Gingrich and Rep. Ron Paul of Texas each got delegates on the ballot in all 18 districts.

Former Sen. Santorum of Pennsylvania, however, failed to qualify in every district and will only be eligible for 44 delegates. He has no delegates running in the 4th, 5th, 7th and 13th congressional districts.

As of January 25, Illinois had 8,272,219 registered voters eligible for the state's open primary, in which voters of any party affiliation can participate. Polls opened at 7 a.m. ET and will close at 8 p.m. ET.

While the top four GOP presidential candidates–Gingrich, Paul, Romney and Santorum–are all on the ballot, Texas Gov. Rick Perry, who suspended his campaign in January, and former Louisiana Gov. Buddy Roemer will appear on the list, as well.

The Democratic primary is also taking place on Tuesday.

In the general election in 2008, then-Illinois Sen. Barack Obama won his home state in a landslide, 59%-35%, over his Republican opponent Sen. John McCain.

- CNN Political Research Director Robert Yoon contributed to this report.

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