Former FBI Director James ComeyJames Brien ComeySaagar Enjeti ponders Hillary Clinton's 2020 plans The shifting impeachment positions of Jonathan Turley Push to investigate Bidens sets up potential for Senate turf war MORE took aim at President TrumpDonald John TrumpTrumps light 97th annual National Christmas Tree Trump to hold campaign rally in Michigan 'Don't mess with Mama': Pelosi's daughter tweets support following press conference comments MORE on Saturday, saying that resignations of staff members on matters of principle must be confusing for Trump, whom Comey says governs "without any external ethical framework." 

Comey's tweet followed the resignations of both Defense Secretary James MattisJames Norman MattisThreatening foreign states with sanctions can backfire Overnight Defense: Erdoğan gets earful from GOP senators | Amazon to challenge Pentagon cloud contract decision in court | Lawmakers under pressure to pass benefits fix for military families Amazon to challenge Pentagon's 'war cloud' decision in federal court MORE and Trump's envoy to coalition forces fighting the Islamic State in Iraq and Syria, Brett McGurk, but mentioned neither.

"To a president without any external ethical framework, folks who resign on principle must be confusing," the former FBI chief, who was fired last year by Trump, wrote on Twitter.

McGurk and Mattis both announced their resignations this week following the president's announcement of a withdrawal of U.S. troops from Syria, and reports that Trump is considering a reduction of U.S. forces in Afghanistan.

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Comey frequently criticized the president and his GOP allies in recent months, and has called for Trump to be voted out of office in 2020.

“I understand the Democrats have important debates now over who their candidate should be, but they have to win,” Comey said at a public appearance earlier this month. “All of us should use every breath we have to make sure the lies stop on January 20, 2021.”

He has also taken aim at congressional Republicans, who he claims are not standing up for law enforcement against the president's criticism of the special counsel investigation.

“Republicans used to understand that the actions of a president matter, the words of a president matter, the rule of law matters, and the truth matters. Where are those Republicans today,” Comey added at a press conference following six hours of interviews with GOP lawmakers on Monday.

“At some point, someone has to stand up and in the fear of Fox News and fear of their base, and fear of mean tweets, stand up for the values of this country and not slink away into retirement.”