Rick Santorum has had a good run. Where does he go from here?

The upset Rick Santorum was hoping for in Illinois didn't come; instead, he lost to Mr. Romney by more than 11 points.

At this point, it's hard to envision anyone but Mitt Romney getting the GOP presidential nomination.

Yes, Mr. Santorum has had a better run than anyone would have predicted even a few months ago. He appeals to conservative Republicans, Evangelicals, and those who really, really want an alternative to Romney.

But the math is not in his favor. In order to get the nomination, Santorum would need to win 69 percent of the remaining delegates – something which simply isn't going to happen.

His other hope – which he and his team have become more vocal about in recent days – is doing well enough to deny Romney the 1,144 delegates he needs, thereby delaying the decision until the August convention.

But that possibility is also incredibly slim, and would require a major screw-up by Romney. Currently, Romney only needs to win 46 percent of the remaining delegates to get to the magic number – not a high hurdle. The implosion of Newt Gingrich's campaign (he got just 8 percent of the vote in Illinois) hasn't helped Santorum the way some predicted, and might make it even harder to keep Romney from steadily amassing delegates.

Moreover, it's not something most Republicans want.

"Whatever slim chances that Mr. Santorum has would depend on going to the floor in Tampa," wrote New York Times polling expert Nate Silver on Wednesday, calling the nomination contest now "a one-man race." "As this becomes increasingly clear to voters, they may come to see their choice as being less one between Mr. Romney and Mr. Santorum and more one between Mr. Romney and a brokered convention. Some of Mr. Santorum’s supporters may desert him once they view the race in those terms, making Mr. Romney’s path easier still."

Add to that Santorum's fundraising deficits: In the past month, his campaign reported raising $9 million to Romney's $12 million, and the Santorum campaign has $2.6 million in the bank compared with Romney's $7.3 million.

And yet, Santorum doesn't sound like he's planning to fold anytime soon.

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