The Secret Service would carry him out. The American bourgeoisie and nat-sec state have no loyalty to Trump and see Biden as a more stable imperialist tool they can use. Besides they need to preserve the illusion of democracy which is more important than any one tool of theirs. If the Republicans cannot rig it in such a way that Trump wins clearly (ballots not being counted or incinerated aside) and they can't finagle up some sort of excuse for the Supreme Court to step in to help them they're done and he's out. Simple as that.