the bottom line of the report that polls on average under-weighted Republicans and Conservatives by 3-4% everywhere, and the final result was worse than in many years before – even as they corrected some of the 2016 errors (stratifying by education). Reagan was the last candidate to be undercounted by as much, and in other races they say delicately “worse in 20 years” with an asterisk that they only track data for 20 years, or, in blunt Trumpian language, “worse polls EVER”. Note that polls were biased towards Republicans in 1936-48 by 4-12% and after that all large outliers 2-8% were in favor of Dems (1952, 64,80,92,96,2020) with an exception of 2012 Obama (2%).
Note that most of the analysis is done on average, linear approximation, without focusing only on elections and states really mattered. So, use conclusions with care