“One of the big existential challenges that we face is predicting who’s going to vote, and projecting out what the electorate’s going to look like is always one of the hardest things any of us as pollsters can do. It’s equal parts empiricism and analysis and feel and emotion. It’s a delicate balancing act,” said Ben Lazarus, a Democratic data expert. “There is some segment of Republicans who probably aren’t answering the phone.”

In some cases, pollsters made incorrect assumptions about the electorate itself. Some pollsters weighted their samples too heavily to Census Bureau figures that skewed more heavily toward young and educated voters. That was especially true in states like Wisconsin, where media pollsters showed Biden running away from Trump while local surveys showed a much tighter race.

“The pollsters who were the most wrong were the ones who put too many controls on their sample frames. If you had an excess of buckets in your frame, all your buckets had a better chance of being wrong,” said Brad Todd, a Republican strategist whose firm conducts polling. “The polls that were the most wrong had too many young people and too many educated people.”