Pew Research: “Looking across the 12 battleground states from the upper Midwest (where many polls missed the mark) to the Sun Belt and Southwest (where many were stronger), polls overestimated the Democratic advantage by an average of about 4 percentage points. When looking at national polls, the Democratic overstatement will end up being similar, about 4 points, depending on the final vote count. That means state polling errors are about the same as in 2016, while the national polling error is slightly larger, at least as of today.”
“The fact that the polling errors were not random, and that they almost uniformly involved underestimates of Republican rather than Democratic performance, points to a systematic cause or set of causes. At this early point in the post-election period, the theories about what went wrong fall roughly into four categories, each of which has different ramifications for the polling industry.”

