Former Joint Chiefs chairman: Trump's review of security clearances reminiscent of Nixon, McCarthy

Retired Adm. Mike Mullen said Sunday that President TrumpDonald John TrumpSchiff urges GOP colleagues to share private concerns about Trump publicly US-China trade talks draw criticism for lack of women in pictures Overnight Defense: Trump to leave 200 troops in Syria | Trump, Kim plan one-on-one meeting | Pentagon asks DHS to justify moving funds for border wall MORE's list of former intelligence officials at risk of losing their security clearances is reminiscent of former President Richard Nixon's enemies list and McCarthy-era politics.

"That historically has proven incredibly problematic for the country," Mullen said on "Fox News Sunday."

Mullen, who served as the chairman of the Joint Chiefs of Staff from 2007 to 2011, said he doesn't support former CIA Director John BrennanJohn Owen BrennanOvernight Defense: Trump declares border emergency | .6B in military construction funds to be used for wall | Trump believes Obama would have started war with North Korea | Pentagon delivers aid for Venezuelan migrants Trump: I believe Obama would have gone to war with North Korea Intel agencies' threat assessment matters more than tiff with Trump MORE being as critical of Trump as he has been, but still doesn't agree with Trump's decision to revoke Brennan's clearance, arguing it infringes on Brennan's free speech. 

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"I think [Brennan] has been incredibly critical of the president, and I think that has put him in a political place, which actually does more damage for the intelligence community, which is apolitical, even as he’s retired," Mullen said.

"I am concerned about the whole issue of free speech, and as long as John is not revealing classified information that he shouldn’t, I certainly think he has the right to speak," he continued.

Trump announced last week that he was revoking Brennan’s security clearance, and is considering revoking the clearances of several other former intelligence officials. Nearly all of the officials Trump named have publicly criticized the president and his administration, prompting speculation that the move was retaliatory.

The White House justified revoking Brennan’s clearance by saying he has made unfounded allegations against the current administration, and claimed he has lied during congressional testimony in the past.

Skeptics have noted that Brennan is among the most outspoken critics of the Trump administration, and frequently lashes out at the president on Twitter. Brennan called Trump's meeting with Russian President Vladimir Putin last month "treasonous."

Dozens of ex-intelligence officials have expressed support for Brennan in recent days and condemned Trump’s decision to revoke his clearance.

Mullen said Sunday that the support of former intelligence officials should send a "strong signal" that revoking additional clearances "would really be off base."