Cohn, a former president of Goldman Sachs, is one of them, and he’s leaving because Trump spurned his passionately argued advice against imposing stiff tariffs on steel and aluminum. If this becomes a pattern, the other grown-ups will leave also. Finding replacements of similar caliber will grow increasingly difficult, and the administration will become increasingly weak.

The grown-ups are the voices of wisdom, realism, reason, and moderation. They don’t seek the spotlight, and they do their work mostly in private, not in public. Every organization needs grown-ups, but they’re particularly essential under the unforgiving glare of Washington politics.

The administration’s pre-eminent grown-ups, besides Cohn, are chief of staff John Kelly, national security adviser H.R. McMaster, Defense Secretary James Mattis, and Secretary of State Rex Tillerson, the former ExxonMobil CEO. McMaster, Mattis, and Tillerson quickly became known in the foreign policy establishment as the “axis of adults.”

None of them has anything to gain by serving in those roles, which are thankless and extraordinarily demanding. (Cohn and Tillerson are independently wealthy.) None of them has to stay, except McMaster, who as an active-duty military officer must obey the commander-in-chief. Yet McMaster is widely rumored to be on his way out, though Trump denies it.

These leaders are critically important to the first U.S. president with no experience in government or the military, nor with any history of involvement with, or even much interest in, government policy. Trump initially filled his administration with people whom he knew but who were poorly qualified newbies – Stephen Bannon, Hope Hicks, Omarosa Manigault Newman, Anthony Scaramucci, Keith Schiller, and others. They’re all gone, despite being Trump favorites, because they had not a clue about operating effectively in Washington.

That’s why Trump desperately needs to keep the grown-ups he’s got and in particular must avoid pushing another one out over a substantive policy position. The Cohn episode is especially egregious because he went down in flames advocating free trade, one of the most mainstream and widely embraced principles in economics. Now who of Cohn’s stature, someone who by definition has nothing to gain by taking the job, will be willing to succeed him? If any of the other grown-ups leave under similar circumstances, who of their stature would even consider those jobs?

In letting Cohn go, Trump is making a leadership blunder. Washington isn’t his world. He needs the best help he can get, like Gary Cohn. Instead, Trump has started down the road of driving – and keeping – the best people away.