The key to interpreting the polls in 2016 involved knowing how voters who say they are undecided or voting for a third party tend to behave on Election Day. Undecided voters tend to break heavily against the incumbent, and in 2016, Clinton was the de facto incumbent seeking to succeed President Barack Obama. Between one-third and half of people who say they will back a third-party candidate normally end up not doing so, and they break heavily back toward the party they normally support. In 2016, the bulk of third-party voters were Republicans unhappy with Trump. In the end, both factors helped give Trump a last-minute surge, a surge that confounded the experts and gave him the victory.

This year is different. Trump is the incumbent, and the undecided factor now weighs against him. If a voter is undecided on Nov. 3, she is very likely to back Biden. There are also few unhappy Republicans planning to throw their vote away this year. At this point in 2016, 7 percent of voters said they would back the Libertarian candidate, Gary Johnson, the former Republican governor of New Mexico. Now, Libertarian candidate Jo Jorgensen barely registers in polls. Indeed, one poll in early June found her getting a mere 1 percent of the vote, on par with what Libertarian candidates received before 2016. There is no secret pool of Trump-leaning voters to save him this go-round.

The polls tell a harsh but largely accurate story. Trump and the GOP are losing because independents who might have backed them have turned away in the aftermath of the pandemic and the killing of George Floyd.