Posting this now to try to head off some of the "see the vote is rigged" talk from Trumpers. Five Thirty-Eight has an analysis of what delays in final vote totals could look like, using Pennsylvania as a likely example. https://fivethirtyeight.com/features/why-pennsylvanias-vote-count-could-change-after-election-night/ They go on to speculate about Trump trying to declare himself the winner in Pennsylvania, and therefore the overall winner, before all the votes are counted. The point is, if there is a "blue shift" after election night, this is the reason why. It's not because the election is rigged, or any shady crap, it's because (in this case) Pennsylvania stupidly would not allow mail-in ballots to be opened and processed before November 3rd at 7 a.m.* So it will take some time to go through them. Any ballots that aren't disqualified are valid, legal votes by people who should have their voices heard. So if Trump and the GOP try to stop the count in Pennsylvania on or after November 4th, they are straight-up trying to steal the election. *Contrast that with Florida, where they've already been processing ballots.
And this is the reason why Trump insists that the winner must be declared on election night. But I don't think he needs any excuses. I think we can look forward to Trump declaring himself the winner on election night, no matter what.