Donald Trump ran for the White House as a change agent hostile to the habits of Washington, the place he nicknamed “the swamp.” It worked. But the customs he continues to upend as president are the scaffolding that supports the otherwise fragile words of our written Constitution. Mr. Trump’s rejection of them is more threatening to both his presidency and our constitutional regime than any technical violation of the law that he has been accused of (at least so far).

Customs are the punctuation marks of republican politics, the silent guides we follow without pausing to consider their authority. They operate in a space that is difficult for formal rules to codify. That the president of the United States speaks with caution and dignity, that he exercises the pardon power the Constitution grants him soberly rather than wantonly, that he respects the independence of law enforcement, and that, to the extent reasonable politics permit, he speaks truthfully — these are all customs, not laws. Law is powerless to impose them and powerless without them.

When Mr. Trump drains language of its normal meaning, the law can do nothing about it. His ridiculing of the United States senator who leads the Foreign Relations Committee, his repeated use of the word “fake” to describe news coverage when he actually means “unpleasant” and his style of rhetoric in front of the United Nations, where he called terrorists “losers” and applied a childish epithet to the head of a nation in whose shadow tens of thousands of American troops serve and with whom nuclear war is a live possibility, are all cases in point. There is no way to formalize conventions of maturity and dignity for presidents. Custom fills that void.

Mr. Trump’s prodigious abuse of language violates the custom according to which presidents use words to convey serious meanings. Examples arrive daily, but here are a few more. The president has promised to decree his way to better health care, which he cannot constitutionally do. He has repeatedly tweeted threats at North Korea whose imprecision has turned red lines into smudges. He zigs and zags between alliances with and attacks on other constitutional officers in such a way that no one can constructively work with him. Yet all of these abuses of language are violations of custom, not law.