There are legitimate criticisms of the dangerous practice of executive branch officials disclosing the details of private presidential conversations with other heads of state. But such questions should be answered by a clash between branches ultimately superintended by the people, to whom the president can also appeal.

Members of Congress, long indifferent to the enlargement of presidential power, appear to be stirring precisely because Mr. Trump has so consistently prodded them. The use of legislators’ institutional powers to impose their will may mark the resuscitation of the separation of powers, which places Congress at the center of the constitutional structure.

Given the history of other expansive empires, it would be unsurprising if the imperial presidency began to retract because it overreached. A shrunken presidency and a revived Congress may be Mr. Trump’s most important, and most welcome, legacy.