New from me: There are lots of unanswered questions about the Postal Service's operations this election year. But both delayed mail delivery & states' unrealistic deadlines for requesting ballots could result in thousands arriving too late in many states https://www.businessinsider.com/post-office-delays-could-skew-early-election-results-towards-trump-2020-8
The swing states of AZ, FL, GA, MI, PA, and WI currently require mail ballots to be received by Election Day, meaning if the ballot arrives 1 day late, it doesn't get counted. There's pending litigation to extend deadlines in some of those states, like PA https://www.businessinsider.com/post-office-delays-could-skew-early-election-results-towards-trump-2020-8
In addition to mail delays, state-level deadlines can set voters up to fail. 32 of the states not sending all voters ballots, including the swing states of GA, MI, NC, OH, PA, and WI, allow voters to request ballots 7 days or less before the election https://www.businessinsider.com/post-office-delays-could-skew-early-election-results-towards-trump-2020-8
Lots of recent national & state-level polling data shows double-digit differences in rates of Democrats vs. Republicans who plan to vote by mail, meaning Democrats could be more likely to have their ballots arrive too late or be rejected for other reasons https://www.businessinsider.com/post-office-delays-could-skew-early-election-results-towards-trump-2020-8
The events of the past week may spook some Dems away from returning their ballots by mail. But there are still likely to be big partisan divides between who ends up voting by mail. And in close contests, a 1 or 2% rejection rate could make a difference https://www.businessinsider.com/post-office-delays-could-skew-early-election-results-towards-trump-2020-8