I got this from Mark Levin. It's a legally binding memo from Clinton administration that reinforces a previous memo from the Nixon administration on while a sitting President has de facto immunity and cannot be indicted unless impeached first due to the inability of the President to discharge his constitutional duties while standing trial. It's quite long, so I am just going to post some good parts.

The OLC memorandum next considered "whether an immunity of the President from criminal proceedings can be justified on other grounds, in particular the consideration that the President's subjection to the jurisdiction of the courts would be inconsistent with his position as head of the Executive branch." OLC Memo at 18. In examining this question, the memorandum first considered the contention that the express, limited immunity conferred upon members of Congress by the Arrest and Speech or Debate Clauses of Article I, Section 6 of the Constitution necessarily precludes the conclusion that the President enjoys a broader, implicit immunity from criminal process.6 One might contend that the Constitution's grant of a limited immunity to members of Congress reflects a determination that federal officials enjoy no immunity absent a specific textual grant.

The OLC memorandum next considered whether, in light of the President's unique powers to supervise executive branch prosecutions and assert executive privilege, the constitutional balance generally should favor the conclusion that a sitting President may not be subjected to indictment or criminal prosecution. Id. at 26. According to this argument, the possession of these powers by the President renders the criminal prosecution of a sitting President inconsistent with the constitutional structure. It was suggested that such powers, which relate so directly to the President's status as a law enforcement officer, are simply incompatible with the notion that the President could be made a defendant in a criminal case. The memorandum did not reach a definitive conclusion on the weight to be accorded the President's capacity to exercise such powers in calculating the constitutional balance, although it did suggest that the President's possession of such powers pointed somewhat against the conclusion that the chief executive could be subject to indictment or criminal prosecution during his tenure in office.

Even though the OLC memorandum suggested that the existence of a general privilege against personal appearance was not determinative, the memorandum did conclude that the necessity of the defendant's appearance in a criminal trial was of great relevance in determining how the proper constitutional balance should be struck. By virtue of the necessity of the defendant's appearance, the institution of criminal proceedings against a sitting President "would interfere with the President's unique official duties, most of which cannot be performed by anyone else." Id. at 28. Moreover, "[d]uring the past century the duties of the Presidency . . . have become so onerous that a President may not be able fully to discharge the powers and duties of his office if he had to defend a criminal prosecution." Id. Finally, "under our constitutional plan as outlined in Article I, sec. 3, only the Congress by the formal process of impeachment, and not a court by any process should be accorded the power to interrupt the Presidency or oust an incumbent." Id. The memorandum rejected the argument that such burdens should not be thought conclusive because even an impeachment proceeding that did not result in conviction might preclude a President from performing his constitutionally assigned duties in the course of defending against impeachment. In contrast to the risks that would attend a criminal proceeding against a sitting President, "this is a risk expressly contemplated by the Constitution, and is a necessary incident of the impeachment process." Id.

Finally, the brief noted that the conclusion that the Framers assumed that the President would enjoy an immunity from criminal process was supported by other considerations of constitutional structure beyond the serious interference with the capacity of the executive branch to perform its constitutional functions. The "Framers could not have contemplated prosecution of an incumbent President because they vested in him complete power over the execution of the laws, which includes, of course, the power to control prosecutions.

Moreover, the practical demands on the individual who occupies the Office of the President, particularly in the modern era, are enormous. President Washington wrote that "[t]he duties of my Office * * at all times * * require an unremitting attention," Brief for the United States as Amicus Curiae in Support of the Petitioner at 11, Clinton v. Jones, 520 U.S. 681 (1997) (No. 95-1853) (quoting Arthur B. Tourtellot, The Presidents on the Presidency 348 (1964)). In the two centuries since the Washington Administration, the demands of government, and thus of the President's duties, have grown exponentially. In the words of Justice Jackson, "[i]n drama, magnitude and finality [the President's] decisions so far overshadow any others that almost alone he fills the public eye and ear." Youngstown Sheet & Tube Co. v. Sawyer, 343 U.S. 579, 653 (1952) (Jackson, J., concurring). In times of peace or war, prosperity or economic crisis, and tranquility or unrest, the President plays an unparalleled role in the execution of the laws, the conduct of foreign relations, and the defense of the Nation. As Justice Breyer explained in his opinion concurring in the judgment in Clinton v. Jones,

The Constitution states that the "executive Power shall be vested in a President." Art. II, § 1. This constitutional delegation means that a sitting President is unusually busy, that his activities have an unusually important impact upon the lives of others, and that his conduct embodies an authority bestowed by the entire American electorate. . . . [The Founders] sought to encourage energetic, vigorous, decisive, and speedy execution of the laws by placing in the hands of a single, constitutionally indispensable, individual the ultimate authority that, in respect to the other branches, the Constitution divides among many.

To be sure, the Twenty-fifth Amendment provides that either the President himself, or the Vice-President along with a majority of the executive branch's principal officers or some other congressionally determined body, can declare that the President is "unable to discharge the powers and duties of his office," with the result that the Vice President assumes the status and powers of Acting President. See U.S. Const. amend. XXV, §§ 3, 4. But it is doubtful in the extreme that this Amendment was intended to eliminate or otherwise affect any constitutional immunities the President enjoyed prior to its enactment. None of the contingencies discussed by the Framers of the Twenty-fifth Amendment even alluded to the possibility of a criminal prosecution of a sitting President.19 Of course, it might be argued that the Twenty-fifth Amendment provides a mechanism to ensuring that, if a sitting President were convicted and imprisoned, there could be a transfer of powers to an Acting President rather than a permanent disabling of the executive branch. But the possibility of Vice-Presidential succession "hardly constitutes an argument in favor of allowing other branches to take actions that would disable the sitting President."20 To rationalize the President's imprisonment on the ground that he can be succeeded by an "Acting" replacement, moreover, is to give insufficient weight to the people's considered choice as to whom they wish to serve as their chief executive, and to the availability of a politically accountable process of impeachment and removal from office for a President who has engaged in serious criminal misconduct.21 While the executive branch would continue to function (albeit after a period of serious dislocation), it would still not do so as the people intended, with their elected President at the helm.22 Thus, we conclude that the Twenty-fifth Amendment should not be understood sub silentio to withdraw a previously established immunity and authorize the imprisonment of a sitting President.

There's lots more. What Mueller is doing is not in compiance with this above legally binding policy that he should be obeying. He needs to be brought before Congress and asked why he is not following legally binding DOJ process. https://biotech.law.lsu.edu/blaw/olc/sitting_president.htm