In the late 20th century and early 2000s, missing some of these voters had not been a big problem, because white college graduates and white non-graduates voted similarly. In 2016, these voters shifted to Mr. Trump, and the polls had failed to capture it.
The report by AAPOR offered an optimistic conclusion. The national polls had been close to correct, overestimating Mrs. Clinton’s vote share by only about one percentage point. That was well within the range of historical polling errors. And there were obvious steps the industry could take to improve in the future, by including more working-class voters or weighting the ones who responded more heavily.
Perhaps most important, the polling association argued, the 2016 experience did not suggest a systematic problem in which polls favored one party. In some years, like 2012, polls slightly underestimated the Democratic share, and in other years, like 2016, they slightly underestimated the Republican share. The report said the direction of those misses was “essentially random.”
The midterm elections of the following year, 2018, initially seemed to support this conclusion. The polls correctly suggested that Democrats would sweep to victory in the House, while Republicans would retain the Senate. State polls were off by an average of about four percentage points, which was historically normal.
The underlying details contained some reasons for concern, though. While polls in some liberal states, like California and Massachusetts, had underestimated the Democrats’ vote share in 2018, polls in several swing states and conservative states, including Florida, Georgia, Michigan and Pennsylvania, again underestimated the Republican share.
For the second time since Mr. Trump’s entry in politics, the polls had somehow failed to reach enough Republican voters in the swing states that decide modern presidential elections. A third election — his re-election campaign — was looming in 2020, and it was one that millions of Americans, both his supporters and critics, would be following passionately.