"The Supreme Court has interpreted the appointments clause as “constitut[ing] a vital power of the President.” Thus, Calabresi stresses, while “inferior officers” may be appointed by “the President alone, by heads of departments, or by the Judiciary,” “principal officers” may only be “selected by the President with the advice and consent of the Senate.” Also, “Congress has specified that the 96 U.S. Attorneys are all principal officers who must be nominated by the President and confirmed by the Senate.”
After analyzing controlling Supreme Court precedent and the circumstances and scope of the special counsel appointment, Calabresi concludes that Mueller is behaving like a U.S. attorney:
"Mueller has acted and has behaved like a principal officer even though he was never nominated by the President nor confirmed by the Senate. In fact, Mueller is much more powerful than is a U.S. Attorney because he has nationwide jurisdiction and can indict foreign citizens and corporations without clearance from main Justice as he did when he indicted more than a dozen Russian citizens and three Russian business entities. This action had a major effect on our foreign policy with Russia. Mueller’s actual powers are greater than those of a U.S. Attorney and are akin to those of an Assistant Attorney General. It is thus crystal clear that Mueller is a principal officer."
"As a “principal officer,” the Constitution’s Appointments Clause required Mueller to be nominated by the president and confirmed by the Senate. He was not. Consequently, Calabresi posits, Mueller’s appointment was unconstitutional and “[a]ll actions taken by Mueller since his appointment on May 17, 2017 are therefore null and void including all of the indictments he has brought, all the searches he has conducted included his phone-logging of Michael Cohen, and all plea arrangements he has entered into.”
thefederalist.com/2018/05/31/3-reasons-rosensteins-special-counsel-appointment-illegal/