More than 80 career national security professionals have signed an open letter of support for Democratic presidential candidate Joe Biden, saying President Donald Trump “has created an existential danger to the United States”.

Most of the signatories, who include career diplomats, intelligence officers and defence policymakers, have served both Republican and Democratic administrations. They noted that their policy views cover a spectrum and that as officials they “have often been in opposition, sometimes bitterly, with each other”.

But in a letter published online Wednesday, they expressed a shared belief that Trump’s approach to leadership has undermined the country’s role in the world.

“His re-election would continue this downward spiral and will likely have catastrophic results,” say the signatories, most of whom have never publicly endorsed a candidate for president.

Doug Wise, a former CIA clandestine officer and former deputy director of the Defense Intelligence Agency, broke a career-long vow to serve in silence by signing the letter.

“We need to restore courtesy, respectability and consensus decision-making based not on the personal interests of Donald Trump but on the personal interests of Americans,” said Mr Wise, who retired in 2016 after 48 years of government service.

Mr Wise, who leans Republican, said he has never voted for president, content to trust in the American democratic system “to produce a good president and commander in chief.” But the system has failed, he said. So this November, he said, he will cast his first vote for president – for Biden.

Larry Pfeiffer, a former senior director of the White House Situation Room and a chief of staff to then-CIA Director Michael Hayden, said he leans Republican. “If Donald Trump wasn’t running, and it was Mitt Romney versus Joe Biden, I’d be endorsing Mitt Romney,” he said. “And I probably wouldn’t be public about it.”

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Mr Pfeiffer, who served five presidents dating back to Ronald Reagan, said he sees himself as nonpartisan, so much so that endorsing a candidate feels like “an unnatural act”.

Margaret Henoch, a former CIA officer who joined the agency in the Reagan administration, agreed that a public endorsement is “absolutely” unheard of for career professionals. But these are not normal times, she said.

Ms Henoch said her endorsement is “not political.” It’s driven by a desire to restore “the stability of the country and the world and the respect for the role and function of government” in a democratic society.

Paul Rosenzweig said he was a Republican but became an independent in 2017 because “the standard-bearer for my party no longer represented the values that I think the party should stand for.”

“Even though I am sure I will disagree with much of what [Biden] does, I am also certain that the overall result will be far superior under Biden than under Trump,” said Mr Rosenzweig, who served as a senior policy adviser at the Department of Homeland Security under President George W. Bush and as a senior counsel to Independent Counsel Kenneth Starr in the Clinton administration.

James Clapper, a former director of national intelligence who entered government service in the Kennedy administration and retired in 2017, has voted “both ways” in federal elections. He considers himself a “Democrat domestically and a Republican in the foreign and national security realm”.

He, too, said he would vote for Biden. “I just think he would represent if elected, a restoration of normality to the country,” said Mr Clapper, a retired Air Force lieutenant general who served in five Democratic and five Republican administrations.