I've read a few articles about polling error leading up to the 2016 election. If you remember, Clinton was up 6 or 7 points going into election day.
From what I read, the populations of big cities were over-represented in the polls, and urban areas tend lean more democrat. I also read that Trump voters were less likely to participate in polls when asked. I wouldn't be surprised if we see something similar in the 2020 polls.
God, I hope this election is not close. Whoever wins, hopefully it's decisive enough to reduce the post-elections-bitching.