Earlier this week, the race for the Republican nomination had one thing going for it: simplicity. Mitt Romney had already won Iowa and New Hampshire — or so we thought at the time — and he was well positioned to cruise to an easy victory in South Carolina. On Tuesday, Nate Silver crunched the numbers and found Romney ahead by double digits in the Palmetto State, giving him a 91% chance of winning of the primary.

This wasn’t four weeks before the primary; it was four days before the primary.

What’s more, Romney wasn’t prepared to let this opportunity slip away. His campaign invested $4.7 million in South Carolina in order to help guarantee success and wrap up the nomination. It looked like it’d be easy — Romney’s principal rival finished fourth in both Iowa and New Hampshire. When was the last time a Republican finished outside the top three in Iowa and New Hampshire, and remained a competitive challenger? It’s never happened.

And yet, as of last night, the simplicity of the GOP race has been replaced with a far more complicated picture.

Newt Gingrich didn’t just beat Romney in South Carolina, he crushed him. Even among those predicting a win for the disgraced former House Speaker, few saw a 12.6-point victory coming. South Carolina has 46 counties, and Gingrich won 44 of them. After losing every congressional district in the state, Romney emerges from this contest with exactly zero delegates.*

Put it this way: in less than a week, Romney managed to turn a double-digit lead into a double-digit defeat, despite an aggressive effort and nearly $5 million in investments. That’s not an easy feat to pull off.

The result is a split we’ve never seen before: three different candidates have won Iowa, New Hampshire, and South Carolina.

Romney doesn’t have any excuses, at least not persuasive ones. He told reporters on Friday, “Speaker Gingrich is from a neighboring state.” If this is meant to add an asterisk to Gingrich’s win, it’s weak — Romney is from a state that neighbors New Hampshire, and he’s actually a homeowner in the Granite State. Does this mean his win there doesn’t count?

The next spin from Team Romney is that losing is somehow a good thing.

A senior Romney campaign official told BuzzFeed Saturday that he expects Newt Gingrich to beat them in the primary here — but argued that the loss will make Romney stronger in November

“I think we’re going to lose tonight, we could lose big,” the aide said, speaking on condition of anonymity. “But I think it’s been a terrible week for Gingrich and a great week for us.”

Um, no. It’s been the worst week of the campaign for Romney, and in the wake of a big defeat, the former governor has to overcome new questions about the strength of his candidacy.

Just how serious a problem is Romney facing? I still believe he’ll be the Republican nominee, and all the pieces are in place for him to compete for the long haul. His financial and organizational edge, coupled with the backing of the party establishment, should carry the day when all is said and done.

But one of the key problems with a defeat as dramatic as this one is the questions that it raises. Romney has been the “inevitable” candidate for a year, and now he’s forced to confront something he’d desperately hoped to avoid: doubt.

* Correction: This was apparently premature — Gingrich won 23 out of 25 delegates, and Romney picked up the other two. The other data points, and the larger point about the one-sided victory, stand.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.