What really saved the Republic from Mr. Trump was a different set of limits on the executive: an informal and unofficial set of institutional norms upheld by federal prosecutors, military officers and state elections officials. You might call these values our “unwritten constitution.” Whatever you call them, they were the decisive factor.
It’s true that the courts at times provided a check on Mr. Trump’s tyrannical tendencies, as with their dismissal of his frivolous attacks on the election and their striking down of his effort to overturn the Deferred Action for Childhood Arrivals program without appropriate process. But in other cases, such as his anti-Muslim travel ban, the courts have been too unwilling to look beyond form to ferret out unconstitutional motive. More generally, Mr. Trump has tended to move fast, while the courts are slow, and to operate by threat, which the courts cannot adjudicate.
The bigger and more important failure was Congress. Madison intended Congress to be the primary check on the president. Unfortunately, that design has a key flaw (as Madison himself realized). The flaw is vulnerability to party politics. It turns out that if a majority of members of at least one body of Congress exhibits a higher loyalty to its party than to Congress, Congress will not function as a reliable check on a president of that same party. This was what happened with Mr. Trump and the Republican-controlled Senate.
The problem is chronic, but over the last four years it became virulent. Confronted with a president who was heedless of rules, Senate Republicans, in ways large and small, let him do what he wanted. They allowed acting appointees to run the federal government. They allowed him to claim a right to attack Iran without congressional approval. The impeachment process was reduced to nothing but a party-line vote. The Senate became a rubber stamp for executive overreach.