The Electorate

There is also a case that Mr. Cruz benefited from a more evangelical Christian electorate than pre-election polls assumed. The entrance polls showed that evangelicals were 63 percent of the electorate, while most pre-election polls showed a much lower tally. The final Selzer poll, considered to be one of the most reliable polls, had evangelical voters at 47 percent of the electorate.

Mr. Trump lost evangelical Christians to Mr. Cruz by a 12-point margin, 33 to 21 percent.

Mr. Cruz would not be the first conservative candidate to outperform Iowa polls. In fact, it’s pretty common. Pre-election polls showed a close race between Mike Huckabee and Mitt Romney; Mr. Huckabee won by nine points. They showed Rick Santorum trailing Mitt Romney significantly; he won, although one could attribute that to late momentum, not a poor measure of the electorate. Pat Robertson outperformed the polls in 1988.

The list of moderate candidates who underperformed is just as long, and Mr. Trump did best among self-identified moderates.

One easy explanation is that the most conservative voters are among the party’s most committed, longtime activists who regularly attend caucuses, and that polls conducted by mainstream media organizations do a poor job of incorporating them into their likely-voter models. That’s because they don’t buy the list of past caucus attendees, which is collected by the state parties and sold at a considerable cost. When asked during a phone survey, voters often misreport how much they’ve participated in the past.

But this is probably not the whole explanation. Mr. Trump, after all, only won 29 percent of the nonevangelical vote — even less than the 31 percent he held overall in pre-election polls. If that’s right, Mr. Trump would have underperformed the pre-election polls, even with a 100-percent nonevangelical electorate.

What’s more, the proportion of voters who were “very conservative” was actually fairly low, at 40 percent of voters — well beneath the 47 percent from 2012. Mr. Cruz won “very conservative” voters by a far greater margin than he did among evangelicals.

And the turnout was, as Mr. Trump might put it, huge. Nearly 187,000 people turned out, or more than a 50 percent increase over 2012 levels. Pre-election polls all showed that Mr. Trump would benefit from a stupendous turnout, but few anticipated a turnout of anything near this level. Even Steve King, the Iowa congressman and Ted Cruz backer, said turnout needed to be around 135,000 people or lower for Mr. Cruz to win. Given the huge turnout, Mr. Cruz might not have even targeted as many voters as he actually won, making it harder to argue that it was his vaunted field operation that put him over the top.