Ulrich said:
All this is predicated on the polling being wrong, but:
I think all the data exists to do really good polling, but it's hard to link it all together, hard to go the extra mile to uncover shy voters, hard to do really good subgroup sampling, and really hard to update a mental model.
If doing things the old, easy way (phone calls with random sampling) gives you the answer you expect, why go to all that effort? The samples aren't random, not really, so acting like they are is going to fail.
You're right that groupthink is a factor, I'm just taking it one step further to try to understand the mechanism by which groupthink without malice applied to theoretically unbiased data turns into a very wrong answer.
I dont think there's much "I want to see Biden +10 so by damn I will find a way to get Biden +10." But I do think that of you do a phone bank you get a bunch of Biden voters because people who pick up the phone and answer pollsters have personality and demographic traits that correlate with blue.
I recall a poster a few weeks ago who stated he was visiting his parents (long-time registered Republicans) and he happened to catch a pollster calling them and decided to play a prank. He proceeded to tell the pollster that his parents had both switched to vote for Biden.
For the next week they were swamped by polling services and they were super pissed at their son because they had to keep telling them they didn't switch. I think he made a joke that he was now out of their will.
It's anecdotal for sure but I think if they catch a registered Republican flipping they put them on speed dial, especially all the Never Trumper's. I also found out recently that many polling services use the same respondents over and over, which to me seems counter-intuitive UNLESS you are driving a certain result.
TL/DR - Polls are biased junk and should never be leveraged again if Trump wins in a landslide.