We now know that only 9% of all calls result in interviews.
Assembling a random sample of the entire universe out of a sample that represents only one-in-eleven is virtually impossible.
Most of the polls weight for the standard demographics -- age, race, gender, income, education, etc. But they do not weight for party identification, simply accepting whatever the result is.
Given the weighting by race, the continual over-sampling of Democrats is obviously an artifact of white Democrats being more likely to answer the phone and respond to a pollster than are white Republicans.
I am wondering who is it that is getting polled? I never answer my phone unless I recognize the caller ID. Otherwise, it goes to voice mail, and I call back if it is someone I want to speak with.