The news today is filled with signs that Rick Santorum is winning in Michigan and is basically tied with Mitt Romney nationally. While Romney is still the favorite for the nomination, the possibility of a long- drawn-out struggle raises another possibility — a GOP nightmare scenario — that has gone unexplored.

Specifically: What if Santorum wins the bulk of contested primaries and caucuses and leads in national opinion polls — but Mitt Romney wins the nomination? If that happens, the general election could get a whole lot tougher for him.

Here’s the scenario. In a close nomination battle, it’s all about delegates. But Santorum’s campaign is badly lagging in organization. This could mean he doesn’t reap all the delegates that might be his due if we assume, for the sake of argument, that his current popularity holds or even gets stronger. Santorum has already failed to make the ballot in two key states, Virginia and Indiana. Has he filed full delegate slates in other states where it’s required? If not, he could win, but still not get delegates.

In most GOP caucus states, the voting is not strictly connected to delegate selection. If Santorum’s voters don’t understand the procedures, it’s very possible he could win the vote and yet pick up only a handful of delegates. Indeed, that may have already happened in caucus states he’s won, like Iowa, Colorado, and Minnesota.

That’s not all.

Because there’s such a bewildering mix of delegate selection rules in the Republican process, a smart campaign can focus on the best places to win delegates. But it’s hard to make good strategic choices when you don’t even have a pollster! We’ll see this on February 28, when Romney will apparently win Arizona’s winner-take-all primary even if Santorum does hang on for a Michigan win, where the delegates are apportioned in a complex mix of rules. It’s very possible that Romney and Santorum could split the two states, giving Santorum great headlines, while Romney cleans up in delegates.

It’s not impossible — though it’s very unlikely — that the popularity contest could leave Santorum as the clear, unambiguous winner, while Romney becomes the clear, unambiguous nominee. Imagine Santorum finishing with a five point edge or more in votes — even as Romney gets crowned the GOP candidate for president.

If that happens, it’s hard to see rank-and-file Republicans accepting the outcome as legitimate, even if the Republican partisan press tries hard to sell it. And it’s even harder to see Romney kicking off the fall campaign successfully if half his party believes he stole the nomination.