Former FBI Director James ComeyJames Brien ComeyCarter Page sues over surveillance related to Russia probe Top Republicans praise Trump's Flynn pardon The new marshmallow media in the Biden era MORE took aim at President TrumpDonald John TrumpTrump alludes to possible 2024 run in White House remarks Trump threatens to veto defense bill over tech liability shield Tiger King's attorney believes they're close to getting pardon from Trump 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 MattisThe Hill's Morning Report - Presented by Mastercard - GOP angst in Georgia; confirmation fight looms Biden under pressure to remove Trump transgender military ban quickly Progressive House Democrats urge Biden against Defense chief with contractor ties 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.”