Retired military generals have described Donald Trump as a “danger” to America’s security as they endorsed Kamala Harris.

On the eve of a critical debate between Ms Harris and her Republican rival, 10 former top US military chiefs released a letter calling the vice-president the only candidate “who is fit to serve” in the country’s highest office.

While Ms Harris had “demonstrated her ability to take on the most difficult national security challenges in the Situation Room and on the international stage”, they wrote, Trump posed “a danger to our national security and democracy”.

The letter, signed by retired General Larry Ellis and retired Rear Admiral Michael Smith, among others, accused Trump of disparaging service members and putting them in “harm’s way”, including with his deal to free 5,000 Taliban fighters.

It coincided with a new Harris campaign advert placed in Palm Beach featuring Trump’s most senior former officials warning of the risks of his White House return.

The attack advert shows a montage of scathing comments about the Republican ex-president by some of his most senior former cabinet officials in what appears to be an effort to goad him ahead of their televised live showdown on Tuesday night.

“In 2016, Donald Trump said he would choose only the best people to work in his White House,” the attack advert’s narrator said. “Now those people have a warning for America: Trump is not fit to be president again.”