Security clearances: What exactly can Trump do?

Top officials sometimes maintain their access so that they can provide requested counsel to their predecessors on classified matters, analysts said. This denial of access will eliminate that as a possibility, national security experts said.

Aside from causing a political backlash, revoking security clearances of former officials who have criticized him also undermines the entire security system -- from a privilege based on status and character -- to a weaponized political tool, security analysts said.

"It is absolutely unprecedented," said former FBI Special Agent Frank Montoya, who has extensive experience in counterintelligence matters. "In the end, the President is the final authority (on security clearances), but it's an abuse of power."

Does Trump have the authority to do this?

The normal process is for the issuing agency to conduct appropriate reviews and make determinations about clearance status. But White House press secretary Sarah Sanders said Wednesday that Trump revoked Brennan's security clearance in large part because of his "erratic conduct and behavior."

The President later told The Wall Street Journal he did so because of Brennan's role in sparking the investigation into Russian election interference and allegations of collusion between the Trump campaign and Russia.

To revoke security clearances, Trump could follow the decades-old executive order in place, which provides a written explanation to the clearance holder and an opportunity to reply. Officials could also try to invoke the "interests of national security" clause, which is found in that section, and avoid the detailed procedures. Finally, Trump could decide he is revoking eligibility for the former intelligence officials unilaterally.

The latter appears to be how Trump proceeded, declaring in a statement that he "exercised my constitutional authority to deny Mr. Brennan access to classified information."

Experts emphasize there is no legal precedent for the President to revoke clearances on his own, as typically revocations would be done by the agency and not for political purposes.

Who is Trump still looking at?

The White House has signaled Brennan will not be the last official to see his security clearance revoked.

Trump said in a statement Wednesday he is still considering revoking the security clearances of several others: former Director of National Intelligence James Clapper, former FBI Director James Comey, former CIA director and National Security Agency chief Michael Hayden, former deputy attorney general Sally Yates, former national security adviser Susan Rice, former deputy FBI director Andrew McCabe, former FBI officials Lisa Page and Peter Strzok, and Bruce Ohr, a current Justice Department official.

The names amount to an unofficial enemies list for this White House. All of those individuals have been the target of Trump's public ire or criticized Trump -- or both.

Despite that, Sanders insisted on Wednesday that Trump's decision to revoke Brennan's clearance and put other critics on notice is not politically motivated.

"The President has a constitutional responsibility to protect classified information and who has access to it, and that's what he's doing is fulfilling that responsibility in this action," Sanders said.

What would change?

National security lawyer Bradley Moss, who routinely represents clients in security clearance disputes, said it's important to distinguish between "access" and "eligibility" to receive classified information. For former FBI officials such as Comey and McCabe -- who were fired -- their access to classified information was terminated when they left government service.

"It's not really a revocation where it's already gone," Montoya explained.

But former top national security officials, such as Clapper and Brennan, have decades of institutional knowledge and security clearances provide them the ability to consult on specific matters with current officials and provide insight if asked.

"It's a red herring to say they're monetizing it -- what senior official doesn't try to write a book?" Montoya added.

What guidelines currently exist?

There are 13 guidelines for clearances, which were established many years ago, says attorney Mark Zaid who adds, they were tweaked during the Bush and Obama administrations to the further benefit of the clearance holders.

According to Zaid, who regularly represents security clearance applicants, not one of the 13 guidelines pertains to political views. The closest one says that anybody who supports the "violent overthrow" of the US government should not have access to US intelligence. Holding an opposing view to any administration does not have any relevance, Zaid said.

What could the long-term effects be?

Steven Aftergood, director of the Federation of American Scientists project on government secrecy, said the move would turn intelligence from a security procedure to a political tool -- one Trump can't exercise without some sort of cause.

"He can't just say, 'I don't like those guys,' or 'They were mean to me,' " Aftergood said. "He would need to specify a particular offense that they committed that would justify revoking their clearance. Saying mean things about the President wouldn't qualify."