"He cannot obstruct justice by exercising his exclusive authority to fire his appointees for any reason or no reason at all. He doesn’t Even have to give a reason. Congress tried twice to restrict that authority by statute. In fact, the impeachment of Andrew Johnson, Lincoln’s VP who succeeded him after Lincoln’s assassination, was because of Johnson’s refusal to snide by the Tenure of Office Act. A subsequent Congressional attempt to restrict the President’s authority was struck down by the Supreme Court as unconstitutional. Congress cannot achieve the same result by trying to bring it under an obstruction of justice statute.
Further, you cannot instruct justice by publicly proclaiming your innocence. Not only is that protected by the first amendment, obstruction of justice requires a corrupt interference (“hey, I didn’t do it and I think the prosecutor is out to get me and the entire investigation is illegal” will not qualify, even if you are guilty. How much more inappropriate to base a corruption claim on that kind of statement if you are in fact innocent. Do we want, as a matter of policy, to base an obstruction charge on protestations of innocence? I think not.
Yes, obstruction can occur even when someone is innocent but, as I said, it must be corrupt. Suborning perjury, bribing a witness, etc., may be obstruction, but nothing that is claimed as a basis for obstruction of justice by Trump fits the bill." -
Mark Pollot, J.D. Constitutional Law & Civil Rights, University of San Diego School of Law (1986)