[D +6 and mid 80s for R support of Trump.]
My feeling is that a lot of these polls are wrong because the people doing them are stupid. Not stupid in the sense of not being able to do simple arithmetic, but too dumb to do this accurately. People who could do polls well exist. But they’re working on Wall Street making millions (or more) each and retiring in their 30’s and 40’s. Because unlike political polls, financial polls (i.e. stock and bond analyses) have tens of trillions of dollars riding on their accuracy.