Yoo said that the Supreme Court has made it “clear” that the president cannot appoint a “principal officer” without getting them confirmed by the Senate.
“The Constitution says that principal officers must go through appointment with the advice and consent of the Senate. In Morrison v. Olson, the Supreme Court made clear that the attorney general is a principal officer. Therefore, Whittaker cannot serve as acting attorney general despite the Vacancies Act (which does provide for him to be acting AG) — the statute is unconstitutional when applied in this way.”
This same argument was made by constitutional lawyer David Rivkin, attorneys Neal Katyal and George Conway, husband of presidential Kellyanne Conway, and Supreme Court Justice Clarence Thomas, who wrote in 2016 that even a temporary appointment of a principal officer that is not confirmed by the Senate would be unconstitutional.