Former FBI Director James Comey James Brien ComeyDemocrats fear Russia interference could spoil bid to retake Senate Book: FBI sex crimes investigator helped trigger October 2016 public probe of Clinton emails Trump jabs at FBI director over testimony on Russia, antifa MORE took aim at 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 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 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 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.

To a president without any external ethical framework, folks who resign on principle must be confusing. — James Comey (@Comey) December 22, 2018

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.

ADVERTISEMENT

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.”