natesilver: OK, I’ll bite. I think an Iowa loss would be pretty bad for Trump. Although maybe not because of “momentum.”

An Iowa loss is bad because the most probable cause of the loss is that his voters aren’t showing up. That means his polls could be inflated everywhere, although probably more in caucus states than in primary states.

The expectations game is harder to gauge, I think, because Trump’s relationship with the media is so oddly codependent.

harry: The question is whether a campaign all about “winning” can take losing. Trump hasn’t lost yet, and the few times he’s gone down in the polls, he’s gone bonkers.

natesilver: He hasn’t lost yet, and he hasn’t really had a moment yet where he had a string of bad polls. It’s interesting too that his top attribute in last night’s ABC News poll was electability. That’s the one where he had the largest lead over his rivals. That contradicts the data showing that Trump is extremely unpopular with general-election voters. But as long as Trump’s winning states and winning in the polls, GOP voters will think he’s a winner, and a great general-election candidate.