(CNN) White House officials have been engaged in preliminary discussions about invoking executive privilege to limit former campaign aide Corey Lewandowski from complying with a congressional subpoena, despite Lewandowski never serving in any role in the administration, according to three sources. House Democrats authorized a subpoena for Lewandowski last month, and served it Thursday.

The White House has invoked executive privilege in the past to block former aides such as Don McGahn from complying with similar congressional subpoenas. While testifying in June, Hope Hicks declined to answer nearly every question about her time in the West Wing, citing instructions from President Donald Trump that she was "absolutely immune" from answering. Annie Donaldson, McGahn's deputy, also did not answer more than 200 questions in her written responses to the House Judiciary Committee, citing similar immunity.

But this would be the first time Trump has tried to invoke privilege for someone who has never worked in the administration when it comes to the Russia investigation. McGahn, Hicks and Donaldson all held titles in the West Wing; Lewandowski has only informally advised Trump since his work on the 2016 campaign ended. The White House once asserted that former Kansas Secretary of State Kris Kobach's conversations with the President about adding a citizenship question to the 2020 census were "confidential." Kobach nonetheless testified in front of the House Oversight and Government Reform Committee.

Trump officials and allies aren't confident the move on Lewandowski will work, skeptical that the President will be able to assert the same executive privilege principles over an informal adviser as he would a staff member. The White House has been in contact with members of the Office of Legal Counsel at the Justice Department about whether it would be successful, and say it remains an option. A White House official cautioned that the discussions are preliminary and no formal Office of Legal Counsel opinion has been sought or rendered by the White House counsel's office yet, though there is one related to Kobach's testimony.

"Executive privilege exists to protect internal government information that, if made public, would cause some damage to the country," said Mark Rozell, the author of "Executive Privilege: Presidential Power, Secrecy and Accountability." "It applies to the President and high-level executive branch officers, not private citizens who never served in the administration."

Read More