"He is sick of mind, impetuous, arrogant, belligerent and dangerous," Gordon Humphrey, who represented New Hampshire from 1979 to 1990, said in a letter to his state's members of Congress obtained by Manchester ABC station WMUR.
Humphrey, who vehemently opposed Trump in last year's election, said the president's comments threatening North Korea with "fire and fury" show that he is "impaired by a seriously sick psyche" and could lead the nation into nuclear war.
In the letter, reprinted in the Concord Monitor, Humphrey wrote:
"The president alone has the authority to launch nuclear weapons, the only restraint being the advice of senior advisors who might be present at the time of crisis, and Donald Trump has shown repeated contempt for informed and wise counsel."
Humphrey said the president's "sick and reckless conduct could consume the lives of millions," and he should be relieved of his duties "at the earliest possible date"
His letter calls on lawmakers to support legislation in the House, HR 1987, which would create a commission to invoke the 25th Amendment and "carry out a medical examination of the President to determine whether the President is mentally or physically unable to discharge the powers and duties of the office."
Humphrey is already receiving pushback.
Neither of the state's two House representatives ― Annie Kuster and Carol Shea-Porter, both Democrats ― support the commission out of concern it would set a dangerous precedent, spokespeople said.
"The congresswoman agrees with Senator Humphrey that the Trump presidency has veered far off the rails," a spokeswoman for Shea-Porter told the Union Leader. "However, she believes creating a commission to pass judgment on the President's mental health sets a potentially anti-democratic precedent."
A local Republican was even more blunt.
"Well, I think the former senator is the one that's mentally unfit," Al Baldasaro, a Trump supporter who serves in the state House of Representatives, told WMUR. "I think he needs to go back into retirement." Baldasaro made headlines last year when he called for Hillary Clinton to be shot to death.
Humphrey supported Ohio Gov. John Kasich during the Republican presidential primaries, but later quit the party in opposition to Trump, and ultimately endorsed Clinton.