• In an open letter released by National Security Leaders for America, 741 bipartisan former national security officials endorsed Harris for president, calling Trump "impulsive and ill-informed."
• The statement was signed by former secretaries of State Hillary Clinton and John Kerry, as well as Republican secretaries of Defense Chuck Hagel and William Cohen.
• The letter criticized the former president's foreign affairs track record and alleged involvement in the Jan. 6th Capitol riot.
• Sunday's endorsement comes as the Harris campaign is in the midst of an effort to rally Republicans who may be losing interest in Trump.

More than 700 former national security and military officials endorsed Kamala Harris for president over Donald Trump in the 2024 election in an open letter published Sunday by National Security Leaders for America.

"This election is a choice between serious leadership and vengeful impulsiveness," said the group, which is made up of bipartisan senior leaders who served in the military and other offices and agencies.

"We are trained to make sober, rational decisions. That is how we know Vice President Harris would make an excellent Commander-in-Chief, while Mr. Trump has proven he is not up to the job," they added.

The statement, which criticized Trump as "impulsive and ill-informed," was signed by 741 people, including vocal Harris-supporting Democrats like Hillary Clinton and John Kerry, as well as Republican secretaries of Defense Chuck Hagel and William Cohen. Hagel and Cohen both served under Democratic presidents.

The signatories include 15 retired four-star generals, 12 former Cabinet-level officials, eight service secretaries, more than 120 ambassadors and three Trump-era officials: Retired U.S. Army Gen. Peter Chiarelli, former Vice Chairman of the Joint Chiefs of Staff retired four-star Gen. Paul Selva and retired Maj. Gen. Eric Thorne Olson.

"Mr. Trump is the first president in American history to actively undermine the peaceful transfer of power, the bedrock of American democracy," the letter said. "He has shown no remorse for trying to overturn the 2020 election on January 6th, promises to pardon the convicted perpetrators, and has made clear he will not respect the results of the 2024 election should he lose again."

The letter also accused Trump of publicly and privately criticizing the leaders of American allies like the United Kingdom, Israel, Australia, Canada and Germany, and criticized his decision to abandon allied Kurdish fighters in Syria in the face of an expected Turkish attack in 2019.

"These are the same people who got our country into endless foreign wars and profited off of them while the American people suffered," the Trump campaign's communications director, Steven Cheung, told CNBC in response to the critical letter. "President Trump is the only President in the modern era not to get our country into any new wars."

Sunday's endorsement comes after 111 Republican former national security and foreign policy officials endorsed Harris in a letter Wednesday, saying that Trump is "unfit to serve again."

Harris' campaign has been seeking to appeal to conservative and moderate voters who may be on the fence with voting for Trump, starting with the launch of "Republicans for Harris" in August. Since then, the vice president has received endorsements from various high-profile Republicans, including Dick Cheney, former vice president for George W. Bush.

"Mr. Trump threatens our democratic system; he has said so himself," Sunday's endorsement letter said. "He has called for the 'termination' of parts of the Constitution. He said he wants to be a 'dictator,' and his clarification that he would only be a dictator for a day is not reassuring."