Laurence Tribe explains all the ways Whitaker's "appointment" is illegal

Yes, Whitaker's Appointment Is Unconstitutional. Here's How To Challenge It. | HuffPost:

[The Attorney General Succession Act] created a line of succession... which would require the position to be filled in this case by the sitting deputy, Rod Rosenstein. It’s hard to imagine what legislative action could be clearer, unless the Senate is expected to pass the law again, now with the added words “and this time we really mean it.”

[Whether or not Sessions was fired] matters because of the Vacancies Reform Act... For the purposes of the VRA, “vacancies” aren’t created by firing. So if a court were to conclude that Sessions actually “resigned,” then Trump and Whitaker could argue that, because of the VRA, anyone working for the Department of Justice could fill the vacancy for up to seven months (210 days).

Yet even if a court were to agree both that Sessions resigned and that the VRA applies ... any federal court will recognize that the Constitution trumps all. And that’s where the big guns get introduced.

The Constitution in Article II permits Congress to empower “the President alone” to appoint “inferior Officers.” All other “Officers of the United States” ― known as “principal officers” ― must be appointed “by and with the Advice and Consent of the Senate.”

The attorney general, however, is a principal officer entitled to Senate confirmation... But Whitaker was never confirmed by the Senate to his new post, nor to any position in the line of succession to that role, or even to any position with similar responsibilities...

So being time-limited – Whitaker serves, as we’ve noted, for at most 210 days – can’t in itself erase the need for Senate confirmation.