The Attorney General of the United States is a principal officer of the federal government. That’s pretty clear: Whoever fills that role is a member of the cabinet, heads a major portion of the executive branch, and reports only to the president.
Because the AG is a principal officer, and because Whitaker has not been vetted by the Senate, Whitaker’s appointment is unconstitutional. Neal Katyal and George Conway III, somehow still husband to Kellyanne Conway, wrote a lengthier New York Times opinion piece arguing just that, quite persuasively. (This doesn’t mean I’ve forgiven Katyal his Gorsuch oped, to be clear.)
Julian Sanchez made the point somewhat more quickly:
Harvard Law professor Laurence Tribe? Same view.
Others are focusing on the Federal Vacancies Reform Act and the DOJ’s specified order of succession, which wouldn’t put Whitaker in the role of acting AG. There’s another solid argument to be made there about the invalidity of Whitaker’s appointment, but the strongest argument is the simplest: His appointment violates not comport with the Constitution’s Appointments Clause.
Every action Whitaker takes as acting AG will be open to challenge. Under any other president, and any other Supreme Court, that’d be hugely significant. It’s less so today, given the Trumpification of the judiciary and the composition of the Supreme Court. This is yet another scenario in which Trump’s banking on his judicial appointees to validate his executive misdeeds.