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The GOP primary has begun to resemble a tennis match in the past month. Mitt Romney will take one victory, Rick Santorum will take the next, then back to Romney. It's game, set, yet the match continues to play out.

Todays' primary in Illinois is the latest round in this ongoing battle. It's an important contest, but regardless of who wins, don't expect the match to end any time soon.

There are five important things to know going into today's primary.

1.) There's a hefty delegate prize.

Sixty-nine delegates are at stake in Illinois. Only 54 of them will be allocated tonight, leaving an additional 15 delegates to be awarded at the state's convention in June.

2.) The popular vote won't affect the delegates awarded, which might cause a problem for Rick Santorum.

Illinois's primary is a bit complex as far as awarding delegates is concerned. The state holds a presidential preference vote, which determines the overall winner of the state. But the vote has no bearing on the allocation of the 54 delegates being doled out today. Instead, voters will directly elect delegates from a list of names on the ballot in their own congressional districts.

This is a disadvantage for Santorum. He failed to qualify for the ballot in four out of Illinois' 18 congressional districts, making him ineligible for a total of 10 delegates. So it will be difficult for him to win the majority of delegates today, regardless of who wins that statewide vote.

3.) Big money has been spent in the state.

Romney and his allies have spent a lot of money on ad buys leading up to the March 20 primary. The Romney campaign has spent $804,852 on ad buys, according to a Republican source tracking media buys in the primary states. That is relatively little compared to the pro-Romney super PAC Restore Our Future, which has spent a little more than $2.6 million on ad buys, according to its FEC disclosures.

By comparison, Santorum has not spent nearly as much. Still, both his campaign and the super PAC supporting him - the Red, White and Blue Fund- have made ad buys. The Santorum campaign has spent $122,532, according to the same Republican source. The Red, White and Blue Fund has spent $310,000, according to its FEC disclosures.

4.) Any registered voter can participate.

There are 8,282,924 registered voters in Illinois, all of whom could, technically, participate in the primary, provided they registered by March 13, and verbally confirm at their polling place that they are indeed a Republican.

5.) Lots of votes have already been cast.

Recent polling shows Mitt Romney with a comfortable lead in Illinois. But that wasn't always the case. Polls showed him in a close race with Santorum a couple weeks ago. Why are these earlier polls worth noting? Because voters in Illinois looking to cast ballots before today had a myriad of options from which to choose, and tabulations from the state elections board show they took advantage of the smorgasbord.

In addition to the standard mail-in absentee ballot, voters could cast in-person absentee ballots, they could vote early or they could vote during something called grace-period voting (which began after and ended before early voting.) According to the state election board, 434,127 votes had already been cast through early voting and grace-period voting.

After the results are determined in Illinois, all eyes will move to Louisiana, where the last primary for the month is scheduled for Saturday.