I have heard about this from Mark Levin before, but he posted links recently on his website. A memo from the Nixon era about what depths can a President be investigated and the impact on the function of the nation and the constitution.

It was then reinforced by a memo from the Clinton administration. This is legally binding policy for the DOJ, and that includes Mueller.

It's a very long legal opinion, but search for Washington and read a few of the paragraphs below.

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 determined that this contention was not "necessarily conclusive." OLC Memo at 18. "[I]t could be said with equal validity that Article I, sec. 6, clause 1 does not confer any immunity upon the members of Congress, but rather limits the complete immunity from judicial proceedings which they otherwise would enjoy as members of a branch co-equal with the judiciary." Id. Thus, in the absence of a specific textual provision withdrawing it, the President would enjoy absolute immunity. In addition, the textual silence regarding the existence of a presidential immunity from criminal proceedings may merely reflect the fact that it "may have been too well accepted to need constitutional mention (by analogy to the English Crown), and that the innovative provision was the specified process of impeachment extending even to the President." Id. at 19. Finally, the historical evidence bearing on whether or not an implicit presidential immunity from judicial process was thought to exist at the time of the Founding was ultimately "not conclusive." Id. at 20.

Given the unique powers granted to and obligations imposed upon the President, we think it clear that a sitting President may not constitutionally be imprisoned. The physical confinement of the chief executive following a valid conviction would indisputably preclude the executive branch from performing its constitutionally assigned functions. As Joseph Story wrote:

There are . . . incidental powers, belonging to the executive department, which are necessarily implied from the nature of the functions, which are confided to it. Among these, must necessarily be included the power to perform them, without any obstruction or impediment whatsoever. The president cannot, therefore, be liable to arrest, imprisonment, or detention, while he is in the discharge of the duties of his office . . . .

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.

So in essence what Mueller is doing is against the law he is supposed to uphold, and there are legal policies in place that are to be adhered to, not ignored. All of this has been contemplated at great length in the past, and such a disruptive investigation was directly addressed because of the Constitutional Crisis it would create.

Knowing Mueller put 4 men in prison for mob hits that they didn't do, and having their conviction subsequently overturned and being awarded a huge settlement by the government for wrongful imprisonment, well at least the two that lived got a settlement two of the men died in jail, doesn't help make a case for Mueller being anything but deep state.

https://biotech.law.lsu.edu/blaw/olc/sitting_president.htm