(CNN) A group of leaders in the Republican foreign policy and national security community wrote an open letter condemning Donald Trump and pledging to oppose his presidential candidacy.

"We are unable to support a Party ticket with Mr. Trump at its head," they wrote in the letter, first posted Wednesday night on the website warontherocks.com

Trump dismissed the letter -- as well as comments by former director of the National Security Agency Michael Hayden who said if Trump ordered torture, the military would disobey him.

"I'm a leader. I don't think that would happen. They're cutting off heads in the Middle East. They're chopping off people's heads in the Middle East and we're afraid to waterboard," the Republican front-runner said Thursday on ABC's "Good Morning America." "Give me a break. Gen. Douglas MacArthur and Gen. Patton are in their graves right now spinning in their graves. They don't believe what they're seeing."

The authors of the strongly worded letter include Michael Chertoff, the second director of Homeland Security under George W. Bush; Robert Kagan, a leader of the neoconservative movement and a member of the Council on Foreign Relations; Eric Edelman, former Under Secretary of Defense Policy during the Bush 41 administration and U.S. Ambassador to Turkey; and dozens of other former Republican presidential administration officials and conservative think tank members.

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