There's a lot of talk that Trump might not leave even if he loses the election. The thinking is he'll claim some sort of fraud and refuse to leave. It's not up to him. The electoral college sends in their ballots. The new Congress declares who the new president is. It is not up to the outgoing president. Article II, section 1, paragraph 3.
Nixon wanted to declare martial law and stay in power during the Watergate era, but he had no support anywhere. His cabinet wanted nothing to do with it and neither did the military. Third world despots do this; they get in power and you can't get rid if them.