Our polls did not overestimate Democrats in the less educated states and districts of the East, where one might expect the phenomenon to show up, though they did underestimate Democrats in California and the Southwest.

Across all polls, some of the misses seem likely to defy the convenient explanations: Late, high-quality polls of Missouri and Florida showed Democratic candidates over 50 percent, for instance, precluding the possibility that late shifts among undecided voters were responsible.

There are other possible explanations.

Some polls still don’t properly represent less educated voters, which was thought to be one of the major drivers of error in 2016. On the other hand, there are plenty of high-quality pollsters that do so and still had very Democratic results in critical states.

The higher-than-expected turnout might have inadvertently contributed to a 2016-like pattern, since lower-turnout voters in the big urban states tend to be nonwhite and Democratic, while lower-turnout voters in rural, less educated states tend to be white working-class voters.

In the Times Upshot/Siena polls, undecided voters tended to follow a similar pattern: In the Sun Belt, the undecided voters tended to be nonwhite Democrats; in the North, they were more likely to be white voters without a degree.

These kinds of errors would actually be acceptable for pollsters. There’s nothing they can do about late movement among undecided voters or an unexpectedly high turnout. And neither error would call the underlying methodology of polling into question.