The mental health of Donald Trump has been under scrutiny since he began running for president. Now 28 Democratic Congress members have signed on to a bill, introduced in April, that could lead to a formal evaluation of his fitness.

The bill seeks to set in motion a part of the 25th Amendment that empowers Congress to establish a body to assess the president’s ability to govern. The commission created by the bill would have 11 members, at least eight of whom would be doctors, including four psychiatrists. If the commission doctors found Mr. Trump unfit to govern and the vice president agreed, the vice president would become acting president. Since the 25th Amendment was written to address temporary disability, it allows the president to announce that he has recovered — presumably Mr. Trump would do so immediately — and force a congressional vote on the finding of unfitness.

The role of psychiatry in this process would be problematic. One of us is a lifelong Democrat, the other a Republican (if an increasingly ambivalent one). But as psychiatrists and citizens, we agree on this point: The medical profession and democracy would be ill served if a political determination at this level were ever disguised as clinical judgment.

Much has been written lately about the Goldwater Rule, the American Psychiatric Association’s prohibition against members’ evaluating anyone they have not personally examined. The rule dates to 1973, when analysis of patients’ unconscious processes drove diagnosis. Today, diagnosis is often linked to observable traits, making evaluation at a distance plausible. Even if Mr. Trump refused to cooperate, diagnosis might be the easy part — perhaps too easy. Whether or not they can say so, many experts believe that Mr. Trump has a narcissistic personality disorder. He is grandiose, entitled, desperate for admiration and so on.