With the benefit of hindsight, it's hard to see how Santorum, Gingrich, or anyone else ever had a chance at the Republican nomination this year. But let's not forget -- people were absolutely freaking out about those possibilities just a few months ago. Romney was the troubled front-runner who had a 30% ceiling and was just barely defeating candidates he was outspending 10 to 1. He was also the candidate who allegedly could not be nominated because of his dalliances with moderation or because of his recent flip-flops.
David Karol has an interesting post at the Monkey Cage in which he argues that Romney's "very inconsistency was a necessary, if not sufficient, condition for his success in capturing his party’s presidential nomination this year." But I think it goes further than Karol suggests. It's not just that Romney had to switch his positions to be a credible potential nominee. I would argue that any Republican presidential nominee today would have to be a serious flip-flopper.
One reason Romney's nomination was relatively predictable was that he was running against the sort of people who are simply never nominated for the presidency by the major parties. Gingrich had a notably unsuccessful and short term as Speaker and hadn't held public office in over a decade. Santorum's initial election to the Senate was somewhat of a fluke and his 2006 drubbing in a swing state did not bode well for him. Bachmann was a member of the House. Cain was an eccentric businessman. Parties almost invariably nominate current or recent senators or governors, and of the prospective field, only Pawlenty, Daniels, Christie, Palin, Perry, and Romney fit the bill. Three of those (Daniels, Christie, and Palin) seemed hesitant to fully jump into the contest, and among the three that jumped in enthusiastically, two of them (Pawlenty and Perry) had serious campaigning problems. Once the two of them had functionally dropped, it was hard to see anyone but Romney getting it, unless it was going to be a Really Unusual Year. And of course you never know whether or not you're in a Really Unusual Year until it's over, but by definition, they're really unusual, so the safe bet is that things are happening as usual.
But here's the key point about that: No one taking the stances Romney needed to take to win this year could have had the sort of résumé needed to be a typical major party nominee. The Republican Party has been moving to the right very quickly in recent years. Almost no one taking the stances that Romney is taking now could have been elected as a senator or a governor from most states just a few years ago. So, if you were consistently conservative (like, say, Bachmann or Santorum), you were either doomed to service in the House or to being kicked out of the Senate. If you had a presidential résumé, conversely, it was probably because your views were pretty moderate a few years ago. Arguably, the only person who can get nominated in the current Republican Party is someone who has pivoted to the right rapidly in the past decade. Rapid polarization makes flip-flopping a necessity.