Fifty prominent Republican national security officials, including a former CIA director, have called party nominee Donald Trump unqualified to lead the country and say he would be "the most reckless president in American history."
The statement was the latest repudiation of Mr Trump's candidacy by veteran Republican national security specialists, and was remarkable for the harshness of its language.
"Mr Trump lacks the character, values, and experience to be president. He weakens US moral authority as the leader of the free world. He appears to lack basic knowledge about and belief in the US Constitution, US laws and US institutions, including religious tolerance, freedom of the press, and an independent judiciary," the statement said.
"None of us will vote for Donald Trump," said the statement, which noted that some signatories also have doubts about Democratic presidential nominee Hillary Clinton.
"From a foreign policy perspective, Donald Trump is not qualified to be president and commander in chief," said the statement, which was first reported in the New York Times.
"Indeed, we are convinced that he would be a dangerous president and would put at risk our country's national security and wellbeing."
The statement coincided with former CIA officer and congressional staffer Evan McMullin - a Republican billing himself as a conservative alternative to Mr Trump - launching a long-shot bid for president.
There is virtually no chance that Mr McMullin - who has no name recognition across the US and isn't that well-known in the Capitol - could win, and only a slight chance he will even be able to get his name on the ballots of key states.
The signatories of the statement, some of whom worked for more than one Republican president, include former Central Intelligence Agency director Michael Hayden, who also headed the National Security Agency; former Homeland Security Secretary Michael Chertoff; former Director of National Intelligence John Negroponte; and two former US trade representatives, Carla Hills and Robert Zoelick.
Other signatories included former senior State Department, Pentagon and National Security Council officials who helped plan and oversee the 2003 US invasion of Iraq. Mr Trump has harshly criticised the Iraq operation, although when radio host Howard Stern asked him in 2002 if he favoured invading Iraq, Trump said he guessed he did.
The statement was organised by Philip Zelikow, who served as a top adviser to former Secretary of State Condoleezza Rice.
The statement did not cite specific comments by Mr Trump, but it clearly was a response to a series of remarks he has made questioning the need for NATO, expressing admiration for Russian President Vladimir Putin, calling for a temporary ban on immigration by Muslims, and inviting Russia to hack Clinton's private email server - which he later said was a joke.
Mr Trump responded with a statement deriding the signatories as members of "the failed Washington elite" who "deserve the blame for making the world such a dangerous place."
"These insiders - along with Hillary Clinton - are the owners of the disastrous decisions to invade Iraq, allow Americans to die in Benghazi, and they are the ones who allowed the rise of ISIS," he continued, using an acronym for the Islamic State militant group.