Washington CNN  — 

House Speaker Nancy Pelosi is demanding acting Defense Secretary Christopher Miller “immediately cease” his plans to install a Trump loyalist as the National Security Agency’s general counsel, calling the move “highly suspect” and arguing it represents a “disturbing disregard” for the country’s national security.

“I ask that you immediately cease plans to improperly install Michael Ellis as the new NSA General Counsel,” Pelosi wrote in a letter to Miller on Sunday. The California Democrat also said she was calling for an probe “into the circumstances of the NSA General Counsel selection process” by the department’s acting inspector general.

“The circumstances and timing – immediately after President (Donald) Trump’s defeat in the election – of the selection of Mr. Ellis, and this eleventh-hour effort to push this placement in the last three days of this Administration are highly suspect,” the letter read.

“Further, the efforts to install him or ‘burrow’ him into a highly sensitive intelligence position 72 hours prior to the beginning of a new administration manifest a disturbing disregard for our national security. Therefore, this placement should not move forward.”

The demand by Pelosi represents the most significant opposition to Ellis’ appointment yet. Her letter came on the same day that the NSA said it was installing Ellis into the post, a move that Miller ordered NSA Director Gen. Paul Nakasone to carry out.

Ellis, who worked for Republican Rep. Devin Nunes of California, one of Trump’s staunchest supporters, before joining the National Security Council, was already facing opposition from Democratic Sens. Mark Warner and Jack Reed. The two had also requested an investigation into the agency’s selection process within days of Ellis being picked for the job last November.

The agency’s selection of Ellis is significant because the role of general counsel at the country’s largest intelligence agency is a civil service position, not a political role, meaning it could be difficult for the Biden administration to remove Ellis.

Pelosi, who wrote that there were “irregularities” in the selection process, noted in her letter that Ellis is “a relatively recent law school graduate with a limited resume, was selected due to interference by the White House, and was chosen over much more qualified candidates.”

“If Mr. Ellis did go through the traditional civil service hiring process, I request a detailed account of that process, to understand how someone with his credentials was chosen over other qualified candidates,” Pelosi wrote.

“The General Counsel of the Department of Defense is the sole selection authority for the position of General Counsel of the National Security Agency - as well as all other senior career DoD General Counsel positions,” according to a Sunday statement from the Department of Defense. “The Director of the NSA does not select or approve of candidates for the position of the NSA General Counsel.”

“To be clear, congressional or media interest in a particular hiring action are not justification under the merit system principles and process to delay placing a selected qualified individual in a position,” the statement added.

CNN’s Manu Raju, Zachary Cohen, Jeremy Herb, Oren Liebermann and Barbara Starr contributed to this report.