A group of 50 former national security officials, all of whom have served Republican presidents from Richard Nixon to George W Bush, have signed an open letter calling Donald Trump unqualified to be president and warning that, if elected, "he would be the most reckless president in American history".
The letter offers a withering critique of the GOP nominee, saying he "lacks the character, values and experience" to be president. The signatories declare their conviction that he would be dangerous "and would put at risk our country's national security and wellbeing". They state flatly that none of them intends to vote for Trump in November. Some have decided to vote for Hillary Clinton, while others intend to sit out the election or write in another name, said John Bellinger III, a former legal adviser to Condoleezza Rice and the writer of the letter's first draft.
"We also know that many have doubts about Hillary Clinton, as do many of us," the letter says.
"But Donald Trump is not the answer to America's daunting challenges and to this crucial election. We are convinced that in the Oval Office, he would be the most reckless president in American history."
In a statement, Trump said the letter writers share the blame for "making the world such a dangerous place". "They are nothing more than the failed Washington elite looking to hold onto their power, and it's time they are held accountable for their actions," he said.
Trump said the former officials - along with Clinton - took part in the decisions that led to the invasion of Iraq, the deaths of Americans in Benghazi, Libya, and the rise of Isis (Islamic State).
"Yet despite these failures, they think they are entitled to use their favour trading to land taxpayer-funded government contracts and speaking fees," he said.
Although no former Secretaries of State signed the letter, it carries the signatures of Michael Chertoff and Tom Ridge, former Secretaries of Homeland Security; Michael Hayden, a former director of the CIA and the National Security Agency; John Negroponte, a former director of National Intelligence and Deputy Secretary of State; Robert Zoellick, who also was a Deputy Secretary of State and president of the World Bank and the US trade representative under George W Bush; Carla Hills, the US trade representative under George HW Bush; and William H Taft IV, a former Deputy Secretary of Defence and ambassador to Nato under the elder Bush.
Also signing the letter were several aides who were senior advisers in the White House, State Department and Pentagon. Among them were Eric Edelman, a national security adviser to then-Vice President Dick Cheney, and Bellinger, who worked closely with Rice when she was Secretary of State and when she was on the National Security Council.
Bellinger said that some involved with the letter wanted to wait until September to release their views but that the candidate's behaviour in recent weeks - from his comments on Nato to inviting Russian intelligence to hack Clinton's emails - galvanised them to move sooner.
"This is not about Nato, it's not about trade, it's not about Russia, it's not about cyber. We really wanted to focus on the character, temperament and judgment that we have seen are required of good presidents," Bellinger said.
Although the signatories all served Republican presidents, many of the criticisms echo those being levelled by the Clinton campaign.