The results will be clear and decisive and he will have no way to dispute them lawfully. So he will dispute them unlawfully.
Trump will call the election rigged, claim it isnt valid and that he is the lawful president. He will say that the deep state stuffed ballot boxes and are trying to overthrow him in a coup. He will prepare his base to take action.
After several days Trump will call for his most loyal supporters to take up arms and come to the WH to protect him. Some small portion of the military and law enforcement, as well as his most fervent supporters, will heed his call. I expect no less than 50,000 and no more than 1,000,000 of his supporters to be ready to take up arms on his behalf.
Shortly thereafter the Democratic house will impeach Trump again and this time the Republican senate will vote to remove him. Im not sure what will happen next as Trump will be surrounded by his supporters in the WH.
It will be a very tense two and a half months until Inauguration Day.
I understand this sounds extreme ... but lets look at the other scenario:
Biden wins decisively and Trump says, Okay, Joe fought hard and won fair and square. Congratulations, I concede.
Honestly, as far fetched as the first scenario is... it seems more likely to me than the second.
Trump has been fanning the flames of his violent, extremist supporters for years. Hes been priming the pump. All he need do is lose and claim the election is rigged for some of them to take action. If he loses, claims the election is rigged AND asks for their help... all bets are off.