Example:
Brexit
Polls showed Remain 52 Leave 48
Vote showed Remain 48 Leave 52
The polls had shorted the leave by 4%
Part of this is not only who was polled but where they lived in the UK. In this election cycle it is not enough to have “X” number of Demonrats and “Y” number of Republicans. They need to look at the totals State by state. If all your Demonrats are in NYC and busy living it up on welfare they may like PIAPS. If they are 45 year-old guys living in the Mid-West who used to work Carrier I sincerely doubt they’ll vote for an open borders candidate
I heard the Internet and smartphone/mobile polls were accurate, though, giving a 1-2% margin for Leave. Very interesting.