Rigged election, voter fraud, and Bigfoot !
He's already declared why and how he lost the election even before he has his first debate.
And he has the same amount of evidence as his claims surrounding the birth certificate (not that it matters to his base of alt-right militias).
Get ready for the first non-peaceful transition of power in history. After Trump leaves the White House (perhaps forcibly) he will immediately start the last chapter of his life trying to start a new civil war. And its not even something based upon ideology - its based upon the same thing he fights with Alec Baldwin about - his ego.