Donald Trump was at the top of each of the last 10 polls in Iowa, but his lead failed to hold up on caucus night Monday. In the end, his seven-point lead in polling averages amounted to a three-point loss to Ted Cruz.

That 10-point swing was enough to make Mr. Trump’s defeat the biggest polling error in an early primary since Hillary Clinton defeated Barack Obama in New Hampshire in 2008. But even that measure understates the extent that the polls misjudged Mr. Trump’s strength.

Mr. Trump was at 31 percent in the final polls, but finished with just 24 percent. In our data set of early primary polls from New Hampshire and Iowa since 2004, no candidate underperformed the final surveys by as much as Mr. Trump. Mrs. Clinton, for instance, mainly beat Mr. Obama by outperforming her polling, not because Mr. Obama fell short.

It’s probably not a coincidence that the candidate who underperformed the polls by the most is also the one who had a mediocre turnout operation and enjoyed seemingly nonstop media coverage.