Mental health professionals have signed an open letter warning that Donald Trump is "incapable of serving safely as president".
The letter, signed by 35 distinguished psychiatrists, psychologists and social workers, said they believe that Trump's "words and behaviour suggest a profound inability to empathise", leading them to draw their conclusion.
Under the Goldwater rule set out in the American Psychiatric Association's code of ethics, it is frowned upon for psychiatrists to give a professional opinion on the state of someone's mental health without examining them in person. However, the doctors felt it necessary to break that rule on this occasion.
Writing to the New York Times, the 35 professionals said "too much is at stake" to remain silent.
"Silence from the country's mental health organizations has been due to a self-imposed dictum about evaluating public figures," the letter read.
"But this silence has resulted in a failure to lend our expertise to worried journalists and members of Congress at this critical time. We fear that too much is at stake to be silent any longer.
"Mr. Trump's speech and actions demonstrate an inability to tolerate views different from his own, leading to rage reactions. His words and behaviour suggest a profound inability to empathise. Individuals with these traits distort reality to suit their psychological state, attacking facts and those who convey them (journalists, scientists).
"In a powerful leader, these attacks are likely to increase, as his personal myth of greatness appears to be confirmed. We believe that the grave emotional instability indicated by Mr. Trump's speech and actions makes him incapable of serving safely as president."
Among the signatories are Dr Lance Dodes, a retired assistant clinical professor of psychiatry at Harvard Medical School, and Dr Joseph Schachter, a former chairman of the Committee on Research Proposals at the International Psychoanalytic Association.