He will chicken out. He'll pull some excuse about unfair rules or a biased moderator that will be completely and utterly transparent to everyone except his base.
It's transparent to his base too. It's not that they don't see the lies, it's that they don't care.
To Republicans, winning matters more than playing by the rules. This isn't some gentlemanly game of chess. It's a knife fight. The future of their country hangs in the balance, and the only thing that matters is being the people who steer the ship. If you have to lie cheat and steal to get there, so be it.
The Dems need to wake the fuck up and start doing the same.