The debate is being driven by two conservative lawyers, ironically the one defending the pro-constitutionality stance being the husband of top Trumpkin Kellyanne Conway, top Trump aide. Calabresi, a legal scholar, seems to argue that Mueller is acting as a principal officer (see the highlighted text before the "but" below) whose broad mandate must be validated by public accountability, including nomination by the Presidency and confirmation by the Senate, that essentially he is independent vs. special counsel as in the 1978 Independent Counsel Act, which sunset in the late Clinton administration. He seems to compare and contrast US attorneys, typically nominated by POTUS, as principal officers to Mueller
Article II Section IIConway contradicts the idea that US Attorneys are principal officers, noting that they report to the AG and in fact under circumstances judges can indefinitely appoint (indefinitely) US attorneys. Mueller is no more than an "inferior officer", whose mandate is specifically limited by statute and subject to a principal officer, the acting Attorney General (given Sessions' recusal) Rosenstein, Rosenstein flatly contradicts that Mueller is operating out of the scope of his mandate in a supervisory role.
He shall have power, by and with the advice and consent of the Senate, to make treaties, provided two thirds of the Senators present concur; and he shall nominate, and by and with the advice and consent of the Senate, shall appoint ambassadors, other public ministers and consuls, judges of the Supreme Court, and all other officers of the United States, whose appointments are not herein otherwise provided for, and which shall be established by law: but the Congress may by law vest the appointment of such inferior officers, as they think proper, in the President alone, in the courts of law, or in the heads of departments.
SCOTUS in fact ruled almost unanimously in favor of the defunct Independent Counsel Act which in fact operated outside the Executive Branch. In fact, one judge (Manafort case) has already dismissed relevant arguments against Mueller.
There is no doubt in my mind that Trump thinks he's above the law and, as others have noted, is more Nixonian than even Nixon. The more he rants against Mueller, Rosenstein, and Sessions, the more guilty he seems to be of impeachable offenses. I'm not a fan of his playing the victim card, and he needs to stop obstructing justice or he's going to be impeached, regardless of the Mueller investigation.
This is one time I disagree with George Will. Trump has no one else but himself to blame for his mess.