President Donald Trump's appointment of his senior adviser Steve Bannon as a member of his National Security Council's principals committee may require the approval of the Senate, but the appointment is so unusual the law regarding the Council has never been tested.

"Obscure law requires Sen confirmation for WH aide like Bannon to serve on NSC," Jonathan Alter, a Newsweek columnist and MSNBC contributor, wrote on Twitter Monday night, pointing to a line in the U.S. code that defines the council's membership.

Breaking: obscure law requires Sen confirmation for WH aide like Bannon to serve on NSC. 50 U.S. Code § 3021 https://t.co/1sRQEnP3CY — Jonathan Alter (@jonathanalter) January 31, 2017

Whether Bannon really requires Senate approval depends on the language of Trump's presidential memorandum naming him to the committee, and whether the principals committee is subject to the same rules as the Council itself.

U.S. Code 50, section 3021, defines the members of the council as the president, vice president, secretaries of state, defense, energy and "the Secretaries and Under Secretaries of other executive departments and of the military departments, when appointed by the President by and with the advice and consent of the Senate, to serve at his pleasure."

Trump's executive order, like directives issued by President Barack Obama and others before him, changes the makeup of the Council.

Unlike Obama, however, Trump specifically added both his chief of staff Reince Priebus and his chief strategist, Bannon, as regular attendees to the principals committee – described in the memorandum as "the Cabinet-level senior interagency forum for considering policy issues that affect the national security interests of the United States" – not to the National Security Council or the Homeland Security Council.

"It doesn't require Senate confirmation to serve on the principals committee, which isn't part of NSC as such," wrote Laurence Tribe , a professor of constitutional law at Harvard University. "The role Bannon has been given is crazy and dangerous but it doesn't seem to violate any law, though it probably should."

Bannon is the former executive director of Breitbart News – an outlet he once described as a "platform of the alt-right " – and his appointment as Trump's chief adviser prompted harsh rebukes from groups such as the Anti-Defamation League and the Southern Poverty Law Center, as well as top Democrats, who called Bannon an unacceptable choice. The alt-right movement is a loosely organized group that developed in response to mainstream conservatism and has been associated with white nationalism and anti-Semitism.

"Steve Bannon sitting on the National Security Council is dangerous and unprecedented. He must be removed," Vermont Sen. Bernie Sanders tweeted Monday . "We need experienced people who will protect our country on the National Security Council, not an extreme right-wing political operative."

White House press secretary Sean Spicer said during his briefing Monday that Obama did the same thing by inviting David Axelrod, a senior adviser, to the meetings.

.@tribelaw Obama did the same thing, by the way. NSC membership is statutory. Attendance is at POTUS discretion. https://t.co/YAzeUai8t7 pic.twitter.com/MXzWyDqQRW — Sean Davis (@seanmdav) January 31, 2017

But Axelrod was never specifically named a member of the principals committee in Obama's 2009 directive . In a column for CNN, Axelrod said he was only invited to observe the meetings.

"I was not a member of the committee. I did not speak or participate," he wrote . "I sat on the sidelines as a silent observer with [former White House press secretary Robert] Gibbs because we would be called upon to publicly discuss the president's decision on that critical matter and the process by which he arrived at it."

Trump's memorandum also removed the director of national intelligence and the chairman of the Joint Chiefs of Staff from the principals committee, instead adding they "may attend where issues pertaining to their responsibilities and expertise are to be discussed."

The administration defended the move, with Spicer on Monday calling it "utter nonsense" to characterize the change as a "downgrade."

"They are at every NSC meeting and are welcome to attend the principals meeting as well," he said, describing the change as a matter of using their time more efficiently.