What Trump Needs to Win: A Polling Error Much Bigger Than 2016’s

Fortunately, most state pollsters now weight by education. There are a couple of exceptions, but they’re generally not polls that get talked about too much anyway. Virtually all of the polling you’re looking at shows white voters without a degree as a very large share of the electorate. They’re just supporting Mr. Biden in far greater numbers than four years ago.

No guaranteed improvement. There’s no reason to assume the polls will be very accurate this year. There’s not even reason to be sure that the polls will be better than they were in 2016, which wasn’t exactly the worst polling error of all time. In fact, the polls were even worse in 2014 and quite bad in 2012 — though few cared, since they erred in understating the winner’s eventual margin of victory. The polls could easily be worse than last time.

Even if the polls do fare better than they did in 2016, they might still be off in ways that matter. In the 2018 midterms, the polls were far more accurate than they were in 2016, but the geographic distribution of the polling error was still highly reminiscent of the error in the presidential election.

Today, polls show Mr. Biden faring best in many of the same states where the polls were off by the most four years ago. Take Wisconsin. It was the highest-profile miss of 2016; now, it’s a battleground state that Mr. Biden seems to have put away.

We won’t know until Election Day whether that simply reflects real strength among white voters, as shown repeatedly in national polls, or whether it’s an artifact of an underlying bias in polls of states. Four years ago, undecided voters broke to Mr. Trump at the end, leading to an error in his direction; today, perhaps they’ve swung back to Mr. Biden.

The survey research industry faces real challenges. Response rates to telephone polls are in decline. More and more polls are conducted online, and it’s still hard to collect a representative sample from the internet. Polling has always depended on whether a pollster can design a survey that yields an unbiased sample, but now it increasingly depends on whether a pollster can identify and control for a source of bias.

Nonetheless, pollsters emerged from the 2016 election mostly if not completely convinced that the underestimation of Mr. Trump was either circumstantial — like the late movement among a large number of undecided voters — or could be fixed if pollsters adhered to traditional survey research standards like weighting by education. If Mr. Trump wins this time, they will be in for a whole new round of self-examination. This time, they might not find a satisfactory answer.