More universally interesting are the reasons Barnes feels so certain the polls will be wrong again.
In its postmortem of 2016 polling, the American Association for Public Opinion Research named three likely reasons why Trump’s support in certain states was underestimated. The first of these is one pollsters always use when they get it wrong: last-moment decisions by undecided voters.
The other two were specific to the 2016 situation. Many polls failed to adjust for the over-representation of college graduates among respondents: “Better-educated people are much more likely to participate in surveys than those with less education,” the postmortem said, and more advanced education was a clear marker of support for the Democratic candidate in 2016.
Besides, some Trump voters failed to reveal themselves as such, preferring to keep their cards close to the chest.