How wrong could America’s pollsters be?

DESPITE POLLS being in essence tied, gamblers betting on the outcome of America’s presidential election are increasingly confident that Donald Trump, the Republican nominee, will win. Polymarket, a prediction market that has seen over $2.6bn traded on the election, gives him a two-in-three chance. Bettors are in effect gambling that polls are underestimating him for the third time in a row.Chart: The EconomistSuch an error is certainly possible. Polling averages show Kamala Harris or Mr Trump leading in each of the seven swing states by a smaller margin than a normal polling error (see chart). Democrats fear there will be a repeat of the substantial polling misses of  2016 and 2020, when Mr Trump did better than expected. But there is no guarantee that the error will be in the same direction this year: pollsters have gone to great lengths to account for previous mistakes. As The Economist’s presidential forecast quantifies, based on historical polling errors, a broad range of…