https://www.theatlantic.com/magazine...oncede/616424/
TL;DR - Trump campaign will file a flood of legal challenges to stop counting ballots after election night. This will cause delays leading to Republican majority state legislatures (in battleground states) to bypass election results and picking Trump loyalist electors, thus giving Trump the electoral college win.
IMO, this is an extreme scenario that I don't see happening. However, if the results are too close, it's really not out of the realm of possibility and nothing says it would be unconstitutional.