A group of 10 retired senior US military personnel who held senior government positions have announced their support for Democratic presidential candidate Kamala Harris.

The military has criticized Republican presidential candidate Donald Trump, calling him a "threat to national security and democracy."

The statement is part of an open letter signed by retired Gen. Larry Ellis and retired Rear Admiral Michael Smith. Former military officials cited Trump's comments disparaging troops as well as his "chaotic approach" to Afghanistan before the US withdrawal, including a deal with the Taliban that allowed the group's 5000 fighters to return to the battlefield.

In contrast, Harris has "demonstrated the ability to deal with the toughest national security challenges in the Situation Room and on the international stage" -- from Russia's invasion of Ukraine to tensions with China in the Indo-Pacific region, military officials said.

"Harris is the best — and only — candidate for president in this race who is fit to serve as our commander in chief," they wrote in the letter, posted on the X social media platform by Harris' campaign staff.

The letter coincided with a new Harris campaign ad released ahead of the two candidates' debate tomorrow night. In it, former Trump aides warn that a second Trump term would create major risks.

"In 2016, Donald Trump said he would choose only the best people to work in his White House. Now those people are issuing a warning to America. "Trump is not fit to be president again, his former aides worry," the ad said.

It includes videos from Trump's former Vice President Mike Pence, former Defense Secretary Mike Esper, former National Security Adviser John Bolton, and former Chairman of the Joint Chiefs of Staff General Mark Milley.

In their letter, the retired military accuse Trump of "failing to take responsibility for his role in putting troops in harm's way." They point to the former president's deal to release Taliban fighters without the Afghan government's involvement and that he left no exit plan after leaving office after losing the 2020 election.