Noam Scheiber:

Right now the only real threat I see to Romney is Huckabee in Iowa. But I think Huckabee would have to win there outright to derail the Romney train (something that’s certainly possible, but not likely). Even a pretty close second by Huckabee still leaves Giuliani third there at best. And, as I’ve said before, that probably freezes the status quo in New Hampshire, which means a Romney win there, followed by likely wins in Michigan and South Carolina.
Actually, there is one other thing that could derail Romney, though I’m not sure how it would play out as a practical matter: All the rival campaigns (particularly McCain and Giuliani) seem to loathe Romney. The feeling seems to be, “If we don’t win, then we at least want that guy to lose.” As I say, I don’t entirely know what the upshot is, but maybe you can imagine some scenario where McCain or Giuliani starts fading and decides to sacrifice himself to take out Romney.
Short of one of those two scenarios, I think Romney’s going to be the nominee.