NPR News

One reason Trump supporters are undercounted could be that Trump disparages pollsters and the media, creating a sense of distrust that could dissuade people from participating in a survey.

“The so-called low-propensity voters, which Trump gambled on showing up, did vote for him,” Miringoff added. “Likely voter models, which incorporated enthusiasm, understated these low-propensity voters. And although it looks like there were an unusually high number of ticket splitters, it was really ‘Bullet Voters’ — folks who voted for Trump and then left. This resulted in Democrats doing better in Senate contests compared to the top of the ticket.”

Another factor that could have thrown pollsters off of Trump’s scent: Trump won late deciders by double-digits.