It seems well established that the President does indeed have the right to preemptively pardon people for crimes they have not yet been convicted of, so that is not my question.
Right now there are news reports that President Trump is asking his staff how his power of pardon works. The speculation by media pundits is that he might use it as a preemptive pardon to derail the investigation by Special Counsel Mueller. Since a Special Counsel is confined to investigating only "criminal matters", if any target of the investigation were to be so pardoned it would seriously hobble certain courses of inquiry.
So my question is: If President Trump used his power of pardon preemptively on a known target of an investigation -- and it was very clear that the reason for doing so was to prevent further investigation (imagine a tweet that same day saying "This ends the unfair and fake witchhunt by Mueller!" -- would that action be able to be considered obstruction of justice under the law?
From the Cornell Law School website:
A person obstructs justice when they have a specific intent to obstruct or interfere with a judicial proceeding. For a person to be convicted of obstructing justice, they must not only have the specific intent to obstruct the proceeding, but the person must know (1) that a proceeding was actually pending at the time; and (2) there must be a nexus between the defendant’s endeavor to obstruct justice and the proceeding, and the defendant must have knowledge of this nexus.
Allow me to provide another, hypothetical, scenario for comparison:
Let's say that a State DA is investigating a public official who has been accused of raping someone. Along the way the DA discovers that a convicted murderer on death row has crucial evidence related to his investigation which would clear the public official of all charges. Unfortunately the man is scheduled to be executed in 3 days so the DA goes to the governor and asks for a temporary stay of execution so he can use the man's testimony to close the case. The poblic official is a political enemy of the governor so he decides to deliberately refuse the request for a stay and the execution goes forward as scheduled, thereby destroying evidence that might have been used for exoneration.
Is the governor guilty of obstructing justice?
This edit is to clarify and prevent excess discussion on something that has started to become a minor discussion thread below. It is certainly relevant to understanding the eventual Answer but it is only tangential to the specific Question asked:
The Office of Special Counsel is an evolution from the Nixon-era caused Independent Counsel Act which was (deliberately) allowed to expire in 1999 due to unanticipated flaws. To replace it a set of Federal Regulations, 28 CFR 600 "GENERAL POWERS OF SPECIAL COUNSEL", was created (also in 1999) by the Justice Department to establish the role of the Special Counsel for cases where the DOJ might be investigating matters that have or appear to have conflict of interest issues.
The tangential matter related to the above Question is whether or not the President can personally fire the Special Counsel. Apparently the President could order the Atty Gen to fire the Special Counsel, but the President cannot directly fire him unless the DOJ was first to go through the involved process of modifying 28 CFR 600.7(d) or alternatively repealing the entire Part 600 set of Federal Regulations.
[28 CFR 600.7] "(d) The Special Counsel may be disciplined or removed from office only by the personal action of the Attorney General. The Attorney General may remove a Special Counsel for misconduct, dereliction of duty, incapacity, conflict of interest, or for other good cause, including violation of Departmental policies. The Attorney General shall inform the Special Counsel in writing of the specific reason for his or her removal."
Former Acting Solicitor General, Neal Katyal, helped draft PART 600 and explains in this article how it was designed to work.