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Donald Trump derided former Defense Secretary Robert Gates, calling the national security official a "clown" and a "mess."

The attacks came after Gates expressed concern about Trump's readiness to be commander-in-chief in a Friday op-ed in The Wall Street Journal. Gates, who served under both Republican and Democratic presidents, wrote that both Trump and Hillary Clinton "have a credibility problem in foreign affairs" before singling out the Republican nominee as "in a league of his own" and "beyond repair" on security issues.

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"Mr. Trump has been cavalier about the use of nuclear weapons," Gates opined. "He has a record of insults to servicemen, their families and the military, which he called a 'disaster.' He has declared our senior military leaders 'reduced to rubble' and 'embarrassing our country' and has suggested that, if elected, he will purge them—an unprecedented and unconscionable threat. As of late, he appears to be rethinking some of these positions but he has yet to learn that when a president shoots off his mouth, there are no do-overs."

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Trump responded via Twitter on Saturday evening, writing that he "never met but never liked dopey Robert Gates."

Never met but never liked dopey Robert Gates. Look at the mess the U.S. is in. Always speaks badly of his many bosses, including Obama. — Donald J. Trump (@realDonaldTrump) September 17, 2016

The Republican continued his attack on the former defense secretary at a Colorado rally later that night.

"We had a clown today, an absolute clown, Robert Gates," Trump told a crowd. "He's supposed to be an expert. He's been there forever ... he goes out and he says negative things about me. I never met him. I never talked to him. Believe me, I am so much better at what he's doing than he is."

Trump campaign manager Kellyanne Conway rejected criticisms against his temperament Sunday on CBS's "Face The Nation."

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"He was defending himself and Mr. Trump has the right to do that," Conway said. "Mr. Trump has the right to defend himself from people who I don't think are looking at the substance of his plans. They're just judging someone they've never met."

GOP vice presidential candidate Mike Pence also weighed in on ABC's "This Week," maintaining that he had "a lot of respect for Secretary Gates" but that the defense official was "just wrong" in his assessment of Trump.

Pence continued to call Trump a "broad-shouldered leader," adding that Gates "was part of the Obama Administration’s handling of the U.S. withdrawal from Iraq," which prompted ISIS to fill a power vacuum.

"I understand that Secretary Gates may not want to hear that but the American people know it's true," Pence said.

Contributed reporting by NBC News' Vaughn Hillyard and Ali Vitali.