Photo illustration: Aïda Amer/Axios. Getty Images photos: Brendan Smialowski/AFP and Jim Watson/AFP

Four years after Donald Trump defied expectations set by pollsters and news organizations, the public should have even less confidence that public opinion data can accurately point to the winner.

Why it matters: This election could be deja vu all over again but worse, with polls setting false expectations amidst voting complicated by the pandemic and a president who has warned of a "rigged" process, the outcome of which he won't accept.

There are three big reasons for this year’s hand-wringing.

1) The problems identified with state polling in 2016 remain. There aren’t enough large sample, quality polls that account for key demographics of voters who tended to vote for Trump, like people without college educations.

  • Many polls done so far this year in swing states like Florida, Michigan and Wisconsin, are “alarmingly” not improved, said Courtney Kennedy, director of survey research at Pew Research Center, who served on an industry panel that published a post-mortem after Trump’s election. "The structural challenges we had in 2016 are still with us."

2) The coronavirus pandemic will make actual voting more volatile.

  • The nightmare matrix includes: the unprecedented surge in mail-in ballots; the U.S. Postal Service’s lack of experience in delivering them on time; the states’ ability to process them; and access to in-person voting on Election Day.
  • For people whose business is to draw a line between asking people who they will vote for and what actually happens, there’s no statistical modeling that can adjust for any one of these, let alone all of them at once.
  • The larger volume of mail ballots could make the vote counts stretch out longer — like the New York Democratic congressional primaries that took six weeks to resolve, said GOP pollster David Winston, who polls for congressional Republicans. “This is going to be happening in a context that we’ve never dealt with before.”

3) We’re still in early innings. Conventions are wrapped up and the campaigns seem like they’ve been going on for years, but there’s a long way to go.

  • There are the unforeseeable, late-breaking events that can change the momentum of the race. Remember then-FBI director James Comey’s letter announcing he was reopening the investigation into Hillary Clinton’s emails?
  • “If ever there has been a year where unforeseeable things can happen, it’s 2020,” said J. Ann Selzer, an Iowa pollster who works with the Des Moines Register. Anybody that’s not wary this time is kind of kidding themselves.”

The big picture: What the polls can tell us now is that Joe Biden has a large national lead over Trump at the moment — possibly large enough that the outcomes in individual swing states could matter less than they did in 2016.

  • What they can’t tell us is whether that lead will hold up between now and November, with all of the unpredictable events that could happen between now and then. This week’s polls are likely to see a bump for President Trump after the Republican National Convention.
  • “What is clear is, Biden is ahead at the moment, and his lead is about twice as big as Clinton’s was at this time in 2016,” said Republican pollster Whit Ayres. “There’s no pollster who’s going to say, Oh, it’s over.”

Between the lines: Biden’s lead in most national polls is anywhere from 7 to 9 points — outside the 3-point margin of past presidential elections that have been close enough to become a state-to-state contest, said Winston, who polls for congressional Republicans.

  • “That puts it right at the cusp of getting it out of state by state. The first challenge for the Trump campaign is to get it back within state by state.”

The bottom line: Pollsters agree that this year there are fewer undecided voters than recent elections. But for some of the most important public opinion gold — swing state voters — there’s the least confidence in getting an accurate reading.

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