Pennsylvania could serve as the catalyst for Trump's refusal to respect the election results and fire up his base into believing that the election was, indeed, "rigged".
Based on what I've read, votes cast via mail there have been sent in huge amounts. However, PA doesn't count these mailed-in votes upon receipt. Instead, they will begin to count them tomorrow, and according to the Mayor of Philadelphia, for example, it could take days for all the votes to be tallied. Thus, the overall winner of PA will likely hinge on all these votes that will be counted post-election day.
Add to this equation the fact that the number of votes mailed in by Democrats accounted for a massive 68% of the 2.3 million returned mail ballots, more than three times the 22% made up by Republicans. (Pennsylvania does not have traditional in-person early voting but does have early in-person absentee voting.)
Because of the disparity in Pennsylvania, Trump is likely to jump out to a large lead on election night as in-person Election Day results, likely to favor him, are posted.
But then, Biden probably will begin erasing the gap as absentee ballots are tallied, which could take days.
Thus, Trump will initially be "winning" PA, but all the mailed-in votes will then swing Biden to winning the state.
As a result, Trump will say, "SEE! I told you. The election was rigged! The Democrats cheated with their fake mailed-in votes!"
He will then contest the election all the way to the Supreme Court, and tell his lemmings that even if the SCOTUS rules against him, the election was 'rigged', and he and they should not respect the election results.
...
I wish this were simple idle speculation and as crazy as it seems on its face, but all the evidence to date about Trump suggests otherwise:
(1) He loves dictators. Putin, Kim Jong Un, Erdogen, etc. All evidence points towards Trump wanting to be a dictator like them;
(2) He has no morals or ethics. Trump doesn't give a rat's ass about his base, and he likely will cling to power at any and all costs;
(3) His largely uneducated base loves him and eats up his Demagoguery. They will believe his allegations of a "rigged" election, and will support his efforts to remain President.
I think this is gonna be an ugly week. Hope I'm wrong.