An open letter signed by 50 Republican national security experts has warned that nominee Donald Trump “would be the most reckless president” in US history.
The group, which includes the former CIA director Michael Hayden, said Mr Trump “lacks the character, values and experience” to be president.
Many of the signatories had declined to sign a similar note in March.
In response, Mr Trump said they were part of a “failed Washington elite” looking to hold on to power.
The open letter comes after a number of high-profile Republicans stepped forward to disown the property tycoon.