If Trump were inclined to overstay his term, the levers of power work in favor of removal. Because the president immediately and automatically loses his constitutional authority upon expiration of his term or after removal through impeachment, he would lack the power to direct the U.S. Secret Service or other federal agents to protect him. He would likewise lose his power, as the commander in chief of the armed forces, to order a military response to defend him. In fact, the newly minted president would possess those presidential powers. If necessary, the successor could direct federal agents to forcibly remove Trump from the White House. Now a private citizen, Trump would no longer be immune from criminal prosecution, and could be arrested and charged with trespassing in the White House. While even former presidents enjoy Secret Service protection, agents presumably would not follow an illegal order to protect one from removal from office.
Although Trump’s remaining in office seems unlikely, a more frightening—and plausible—scenario would be if his defeat inspired extremist supporters to engage in violence. One could imagine a world in which Trump is defeated in the 2020 election, and the immediately begins tweeting that the election was rigged. Or consider the possibility, albeit remote, that a second-term Trump is removed from office through impeachment, and rails about his ouster as a coup. His message would be amplified by right-wing media. If his grievances hit home with even a few people inclined toward violence, deadly acts of violence, or even terrorist attacks against the new administration, could result.