What if there's ballot fraud making a third party candidate win some electoral votes to keep the race closer?
Third parties don't have enough money to pay the on-the-ground operatives - so that can't happen.
It's no longer possible under the US system to win any significant office without the backing of one of the two major parties. Too many millions of dollars are needed. When both parties become corrupt - as they currently are - then it's almost impossible for an outsider to break in. Look at all of the screaming over semi-outsider Trump - because he took out Empire loyalist Clinton. Tulsi Gabbard and Kyrsten Sinema were practically flung out of the Democrat Party for talking about the way things really work, and the Republicans have done the same to quite a few of their representatives. When Thomas Massie (R - Kentucky) pointed out recently that every Representative has an AIPAC "minder" the GOP leadership just about melted down. Those secrets are not supposed to be discussed.
Independent thinking is not allowed in American party politics - express opinions that deviate from the Empire's plans and you will be out of a job very quickly. Trump has remade the Republican Party to a certain extent by throwing out the Empire's warmongering old Bush and Cheney families, but there is still much work to do.