Several law scholars have made cogent arguments the appointment was flawed. Lower court cases affirming constitutionality have been questioned, including Steven Calabresi, one of the founders of the Federalist Society and a law professor at Northwestern.
Calabresi argues that under the Appointments Clause of the Constitution, an official overseeing the probe would be a "principal officer," and thus would have to be someone who was appointed by the president and confirmed by the Senate.
"[Mueller] is not a principal officer, and since he has not been nominated by the president and confirmed by the Senate, everything he has done since May 1, 2017 has been unconstitutional and has been illegal," he said.
Theoretically, Mueller reports to Rosenstein, since Attorney General Jeff Sessions recused himself from the investigation. But even though Rosenstein was confirmed by the Senate – making him a "principal officer" – Calabresi argues that he's not supervising Mueller's work closely enough to count under the Appointments Clause.
The stakes of the argument are high. If Mueller's appointment really is unconstitutional, then Calabresi argues all of the actions he's taken since his appointment in May of 2017 are "null and void," including "all of the indictments he has brought, all of the plea bargains he has entered into, all the searches he has conducted, his phone logging of Michael Cohen and subsequent of referral of Michael Cohen to the Southern District of New York for prosecution, and any other governmental actions he has taken."
"With all due respect to the statute and to the CFR [Code of Federal Regulations], I think Mueller is much too important an officer to be called an inferior officer," he said. Since his appointment more than a year ago, Mueller has indicted 19 people and secured five guilty pleas. It's all, according to Calabresi, the "fruit of a poisonous tree."