Measuring the Trump vote: a years-long headache for US pollsters

The Republican's victory over Democrat Hillary Clinton in 2016 took the United States and the world by surprise. But it was above all the 2020 election, won by Joe Biden, that forced a reckoning for pollsters. That vote produced polling errors of "unusual magnitude" -- the highest in 40 years for the popular vote, and 20 years for state-level estimates, according to a report by the American Association for Public Opinion Research (AAPOR). Right up until the eve of the election, and despite corrective measures taken in the wake of 2016...