WHAT THIS MAY COME TO

I'm becoming convinced that in a true crisis he would be deposed by a military coup. It's the only thing that would be fast enough. A less drastic form (but more legal form) would be refusing to obey an illegal order. But Trump could, of course, always find someone who would obey it (the way Nixon kept firing people until he got to Robert Bork, who obeyed him, during the Saturday Night Massacre).

Per Seymour Hersh, such a coup was contemplated before Nixon resigned in August '74.

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[April '74] In April of 1974, Joseph Laitin, a public-affairs official who had served in the Johnson White House, telephoned Schlesinger. Although Laitin was a liberal Democrat, the two had become friends early in the Nixon Administration, after Laitin was reassigned as a press official in the Bureau of the Budget, where Schlesinger was in charge of analyzing defense and intelligence programs. They had remained close as Schlesinger moved up in the government—to chairman of the Atomic Energy Commission in 1971, director of the Central Intelligence Agency in February of 1973, and to the Pentagon in May. Laitin broached some of his fears: Was it possible for the President of the United States to authorize the use of nuclear weapons without his secretary of defense knowing it? What if Nixon, ordered by the Supreme Court to leave office, refused to leave and called for the military to surround the Washington area? Who was in charge then? Whose orders would be obeyed in a crisis? "If I were in your job," Laitin recalls telling Schlesinger, "I would want to know the location of the combat troops nearest to downtown Washington and the chain of command." Schlesinger said only, "Nice talking to you," and hung up.

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[Late July '74] Bearing that in mind, and aware that Brown [Chairman of the Joint Chiefs - Bill B] had taken an oath of office that made him responsible to Nixon as Commander-in-Chief, Schlesinger trod delicately during their talk. His goal was to express his concerns about the White House and somehow to get Brown to reach the same conclusion that he himself had already reached. In essence, Schlesinger asked Brown for a commitment that neither he nor any of the other chiefs would respond to an order from the White House calling for the use of military force without immediately informing Schlesinger.

The Pardon (The Atlantic)