Is it legal for the President of the USA to direct an official of the DOJ (say, the Director of the FBI) to drop an investigation, as an exercise of prosecutorial discretion?
This is the case being made by Andrew McCarthy:
[T]he president absolutely has the authority to exercise prosecutorial discretion.
A legitimate exercise of executive power cannot be corrupt. A president does not corruptly impede an investigation by deciding that the equities weigh in favor of halting it. That is a decision the president gets to make. Source
Clearly, there is some special pleading going on there. But I am wondering whether Mr. McCarthy is correct on a point of law.