Ezra Klein skrev på Vox.com för en månad sedan om varför han inte tror Donald Trump vinner den republikanska majoriteten i Iowa:
Candidates lead, and then they don't. Eric Cantor is invincible, and then he's beaten by some unknown academic. Hillary Clinton is a juggernaut, and then she's beaten by Barack Obama. Howard Dean leads everywhere, and then he leads nowhere.
Political analysis is a realm of post-hoc storytelling. Something unexpected or unpredictable happens, and then we explain why it was obviously going to happen all along.
But this is, I think, what will happen to Trump. He will lead until he doesn't. His fall will be quick, and it won't obey the apparent rules of his rise. If there is a reason for it, it will fundamentally be, "People get more pragmatic the closer they get to an actual vote."