Since it is unlikely that all of the mail-in ballots will be counted in all states, it is doubtful that a meaningful projected winner can be declared on election night. Nothing in the statutes suggests that a winner must be declared on election night. The projections are simply a news network feature (sort of like a sports broadcast) to improve their ratings. You can expect Trump to declare some sort of "fraud" and send his vigilantes out to impound anything that they can touch. Anything impounded will disappear behind some sort of executive privilege. Of course resolving this mess will require weeks or months and Trump will claim that he should remain president in the interim. In terms of the constitution, he will be bounced on January 21 and the then Speaker of the House will become President. This should be a nightmare prospect for Trump and his minions.
Imagine the blow to Trump's ego if the Senate flips and the House remains Democratic as he is bounced on January 21 and looses his executive privilege.