Throughout his tenure, Mr. Trump has rarely needed to exceed the limits of executive power in order to abuse his office. Rather, he has inflicted damage again and again by personalizing and misusing powers that the presidency clearly possesses. That is what happened in the Ukraine affair, which is a scandal less because of the specific acts Mr. Trump took than because of why he took them.

As president, from the moment he swore an oath to “faithfully execute the office of the president of the United States” and to “preserve, protect and defend” the Constitution, Mr. Trump has never pretended that he serves an interest larger than his own. Rather, he boldly asserts that there is no public good distinct from his personal, political and financial interests — that what is good for him and what is good for the country are indistinguishable. This is why he seems unable to fathom that he did anything wrong in the Ukraine affair: His attempt, by leveraging military aid, to force an embattled country’s president to investigate one of Mr. Trump’s political opponents was business as usual.

This personalized vision of his office and its power is why Mr. Trump openly embraces deploying law enforcement authorities against his political enemies. Law enforcement should go after the bad guys and protect the good guys. But to Mr. Trump, the “good guys” are his friends and the “bad guys” are his enemies. In the earliest days of his administration, that view led him to ask his F.B.I. director to drop an investigation of his national security adviser and to demand that the attorney general investigate Hillary Clinton. With the Ukraine affair, the impulse is merely exported to the arena of foreign policy, where the president is even less legally constrained than he is domestically. If it is legitimate to use the awesome law enforcement powers of the American federal government to serve Mr. Trump’s interests, why shouldn’t he expect a foreign leader to do the same?

In Mr. Trump’s highly personalized presidency, foreign policy is a matter of impulse and whim, with no concern for facts or national interests. As fateful a step as the killing of an Iranian military figure can take place because the president, upset about leaks and negative press coverage, wants to look stronger than his predecessor. When President Recep Tayyip Erdogan of Turkey suggested to Mr. Trump that United States forces could withdraw from Syria, the president was happy to oblige without any significant consideration of the consequences — even announcing it on Twitter. But when the newly elected president of Ukraine requested a meeting at the White House, Mr. Trump required “a favor” first.

In every policy arena, the president’s traditional management functions — as chief executive of a giant organization — have been subsumed by Mr. Trump’s desire for self-expression and impulsive announcements of his desired outcomes. In issuing the hastily written travel ban in his very first week in office, Mr. Trump chose to push aside the departments of State, Justice, Defense and Homeland Security. Those departments then had to spend months contorting themselves to claim that a travel ban affecting Muslims was not the Muslim ban that candidate Trump had promised.