A petition signed by 350 psychiatrists and other mental-health professionals claiming that President Donald Trump's mental health is deteriorating rapidly amid impeachment proceedings is set to be submitted to Congress on Thursday.

"We are convinced that, as the time of possible impeachment approaches, Donald Trump has the real potential to become ever more dangerous, a threat to the safety of our nation," said Drs. Bandy Lee, a Yale psychiatrist, Jerrold Post, a former CIA profiler, and John Zinner, a psychiatrist at George Washington University.

The petition was first reported by the British news outlet The Independent.

Lee told The Independent that Trump appeared to be showing signs of delusion by doubling down on falsehoods and conspiracy theories.

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A group of 350 psychiatrists and other mental-health professionals are set to submit a petition to Congress on Thursday claiming that President Donald Trump's mental health is rapidly deteriorating amid the impeachment inquiry.

"We are speaking out at this time because we are convinced that, as the time of possible impeachment approaches, Donald Trump has the real potential to become ever more dangerous, a threat to the safety of our nation," Drs. Bandy Lee, a Yale psychiatrist, Jerrold Post, a former CIA profiler, and John Zinner, a psychiatrist at George Washington University, wrote in a statement accompanying the petition, which was first reported by the British outlet The Independent.

They wrote that "failing to monitor or to understand the psychological aspects" of impeachment on Trump "or discounting them could lead to catastrophic outcomes."

They outlined the facets of Trump's behavior that prompted them to speak out:

"What makes Donald Trump so dangerous is the brittleness of his sense of worth. Any slight or criticism is experienced as a humiliation and degradation. To cope with the resultant hollow and empty feeling, he reacts with what is referred to as narcissistic rage.

"He is unable to take responsibility for any error, mistake, or failing. His default in that situation is to blame others and to attack the perceived source of his humiliation. These attacks of narcissistic rage can be brutal and destructive."

Dr Lee told Business Insider by email on Friday morning that the number of signatories had risen to 773. She said that she was prepared with two other psychiatrists to consult with lawmakers about the effect of impeachment on the president's mental health.

"We implore Congress to take these danger signs seriously and to constrain his destructive impulses," they wrote. "We and many others are available to give important relevant recommendations as well as to educate the public so that we can maximize our collective safety."

Lee first publicly expressed concerns about Trump's mental health shortly after he took office in January 2017. The move was controversial — some said it violated the American Psychiatric Association's (APA) Goldwater rule, which states that psychiatrists should not make public statements about the mental health of public figures unless they have personally examined them.

It does not appear that the three psychiatrists have personally examined Trump.

Republican Sen. Barry Goldwater, who ran for president in 1964 and lost to the Democrat Lyndon B. Johnson. AP Images

The rule was adopted after a controversial Fact magazine article in 1964 outlined several psychiatrists' claims that Barry Goldwater, then a Republican presidential candidate, was not mentally fit to become president.

Lee told Business Insider that they were not breaking ethical rules for psychiatrists, because of a clause in the APA rules obliging psychiatrists "to educate the public when asked about a public figure, so that we may improve the community and better public health."

She said "whenever the Goldwater rule is mentioned, we should also refer to the Declaration of Geneva, established by the World Medical Association 25 years earlier, which mandates physicians to speak up if there are humanitarian reasons to do so. This Declaration was created in response to the experience of Nazism."

It's not just medical professionals who have voiced concern about Trump's mental health.

A recent book by an anonymous Trump administration official claimed that White House staff members were concerned about the president's mental acuity and said he "stumbles, slurs, gets confused, is easily irritated, and has trouble synthesizing information, not occasionally but with regularity."

Lee told The Independent that she expected that Republicans would dismiss the petition as colored by partisan bias, but she insisted it was based on observations and accounts of Trump's behavior.

She told The Independent that Trump appeared to be "doubling and tripling down on his delusions," based on his "ramping up his conspiracy theories" and "showing a great deal of cruelty and vindictiveness" in his "accelerated, repetitive tweets."

"I believe that they fit the pattern of delusions rather than just plain lies," she said.