There is uncertainty about the accuracy of polling in certain swing states, the efficacy of GOP voter suppression efforts and even the number of mail-in ballots that for one reason or another will be disqualified.

“There are more known unknowns than we’ve ever had at any point,” said Tom Bonier, CEO of the Democratic data firm TargetSmart. “The instruments we have to gauge this race, the polling, our predictive models … the problem is all those tools are built around quote-unquote normal elections. And this is anything but a normal election.”…

Flagging “clear warning signs” for Biden, one prominent strategist circulated a memo among Democrats earlier this month citing increasing registration of white, non-college educated voters — Trump’s base demographic — in Pennsylvania, Wisconsin and Michigan. There is no precedent for Trump overcoming such a large polling deficit this close to the election, the strategist wrote. “And yet …”

For Democrats, uncertainty about who is voting is only part of the equation. It’s also unclear how many mail-in ballots ultimately will be rejected, and in which states.