The Illinois primary is a lot more complex than people give it credit for. The rules governing the primary are the same as last year's and the state has what is referred to as a loophole primary – it is not strictly proportional and it is not winner take all. It's a weird hybrid.
Here's the way it works (another great explanation here).
Members of the state will actually end of having a say in the allocation of 69 delegates, but the mainstream press is (correctly) reporting that 54 delegates will be picked tonight. However, the way they are picked is a little tricky. Each voter is actually checking off boxes in two separate processes. In fact, you can look at a sample ballot here:
In one vote, the primary voter is selecting one of the following six people: Mitt Romney, Newt Gingrich, Rick Santorum, Ron Paul, Buddy Roemer, and Rick Perry. This choice is the voter's presidential preference vote, and it will likely say something about which way the delegates go, but as a matter of legal obligation, it has no bearing on who gets the delegates from the state.
Rather the delegates (between 2-4 in each Congressional district) are chosen directly (again, look at the ballot to see how this would work, it's incredibly helpful) and each one has an affiliation next to him or her. So a delegate choice might look like this:
Pick at most 2:
Nancy Brandt (Paul)
Tim Stetson (Paul)
Kay Ferris (Santorum)
Andrea Zinga (Romney)
In this toy example, Ms. Brandt is allied with Ron Paul, but it does not mean she is legally required to vote for Paul.
So, one could vote for Mitt Romney for president but then vote for two Paul delegates and two Romney delegates. If everyone in the state did that, Romney and Paul would split the delegates even though Romney would win 100% of the “for president” vote.
Not only that, the two delegates for Romney in my example could change their mind later and vote for Ron Paul at the national convention. Again, keep in mind that Ron Paul is positioning delegates who might change their mind. So, even if Romney “gets” all the delegates, they could all vote for Ron Paul. Of course, they likely won't do that, but what if Romney slips behind Santorum and these delegates are faced with the choice of either voting for a failing Romney, the obviously unelectable Santorum, or the consistent and steadfast Paul? Some more Paul votes don't look so ridiculous. In essence, the delegate system allows a late-game abandonment of failing candidates, so it's possible that a seemingly successful Romney candidacy could well collapse later on.
In Illinois currently though, Romney will likely prevail. Santorum is not in the ballot in some districts, meaning that he'll give up 10 delegates to Romney no matter what, and he'll likely lose more to the centrist and more urban Chicago Republicans. Democrats however can vote as the primary is open, and they are continuing the trend of voting for Santorum. Here's the WSJ catching up with one such democrat: “Sheli Rosenberg, a 70-year-old Democrat, picked a Republican ballot and then selected Mr. Santorum for "pure mischief," she said. "I would like him soundly defeated in the national election if possible so his whole ideas [sic] could get trashed.”
So, what does all this mean for whether Romney's win will end the GOP race? I think it means that the race will continue. The votes that Santorum gets for delegates, even as he likely loses the “preference” vote are the nearly-silent cries of doubt from the primary goers. They are his justification to stay in the race and try to convince delegates to change their mind later, if a a brokered convention ends up happening.
The real thing to watch for is the ratio of votes for Romney in the more clear cut “preference” election and the number of delegates he gets. A large discrepancy means people are uneasy.