If mail in voting is widespread, early returns could be Trump friendly.

Think about it, let's say a lot of swing states have mail in ballots along with normal in person voting. The in person votes will be tabulated by the end of election night, while the mail in ballots could take weeks to count. It's been documented pretty extensively that the vote in person crowd is heavily conservative while the mail in voting crowd (not absentee ballots, but preferring mail in voting) trends more liberal. Of course, the media would give Biden all the cover he needs to drag it out for weeks or months to see how many votes they need to discover in car trunks. To me that's the most frightening part, they can know how many mail in ballots they need to flip a state after election night.