Not sure why anyone thinks Romney is in better shape today than last week. His national numbers have slid down a little & IL changed little.What I'd say is that Romney pretty much ended the possibility of one very real nightmare scenario with his solid Illinois win. No, not a contested convention: I don't think he was in any danger of that, and hasn't been since Florida (or, really, South Carolina). Winning ugly, however, was still at least a plausible outcome: Romney wins the nomination by maximizing delegates/vote despite having week after week of "losing" according to news coverage.
Suppose, for example, that Santorum had edged out Romney in the beauty contest vote in Illinois, while Romney took advantage of better organization to win a small majority of the delegates. That was a very plausible, in my view, outcome, but it still would have left Santorum no closer to the nomination (especially after Puerto Rico). Add a few more weeks to that, and it was possible that Romney's nomination might have looked like a theft to many conservative voters and activists.
That's just a lot less likely now. There's also virtually no chance that Santorum will be able to force another debate, or even manage to make Romney look bad for avoiding one. Nor is Santorum likely to raise much money going forward.
Yes, for the most part the recent primaries have been predictable based on demographics. But pushing all of that a few points in either direction was surely possible, and a few points in Santorum's direction -- while, again, not even slightly threatening Romney's grip on the nomination -- could have made things ugly. Now, it's still possible to imagine even a handful of upsets in the waning days of the race, but a really embarrassing string of losses (while still piling up delegates) is not going to happen, and indeed it's far more likely at this point that if there are any shifts they'll be towards Romney.
How much of a difference any of this makes beyond June was never very clear. But to the extent it did matter at all, Illinois put a major dent in the possibility of it.