Former conservative radio host and commentator Charlie Sykes says a primary challenge from outgoing Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE would be President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE’s “worst nightmare.”

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“Can you imagine if there was a primary challenge of Donald Trump by General Mattis or if Democrats put somebody like Gen. Mattis on the ticket?” Sykes said on MSNBC's "Morning Joe."

“This would be the worst nightmare for Donald Trump because he has portrayed himself fraudulently as the strong man who backs the military, and basically Gen. Mattis is in the process of calling B.S. on the president.”

Mattis resigned last week from the administration in protest over Trump's foreign policy after the president decided to pull U.S. troops from Syria.

In his resignation letter, Mattis wrote that Trump deserves a Defense secretary "whose views are better aligned with yours" and said he was concerned with how Trump treats U.S. allies.

Trump on Saturday hit back, saying that he had given Mattis a "second chance" after he was forced out of the Obama administration. He also said that Mattis would leave by January, pushing him out two months early.

Mattis's exit has sparked speculation over his future plans even though the retired Marine Corps general rebuffed attempts in 2016 to run as an independent against Trump, according to The Washington Post.

Sykes, a longtime critic of the president, went on to blast Trump, saying there appeared to be “no bottom” to his lack of “decency.”

“Up until now we’ve seen that there seemed to be no bottom,” he said. “There seemed to be no limit and I don’t know if this administration is terminal but there’s no question about it that there’s really fundamental cracks in the way he’s handling Mattis.”