Procedurally, whenever a president issues a pardon to abort a criminal conviction, terminate criminal charges, or even prevent prospectively criminal charges from ever being brought, a judge typically has to issue a formal order dismissing the matter. The prosecutor certainly could condemn the pardon publicly as a “cover-up,” as Independent Counsel Lawrence Walsh did in 1992 after the first President Bush pardoned six persons, including former defense Secretary Caspar Weinberger, in the Iran-contra scandal. But Walsh never sought review by a court of the validity of the pardons as he theoretically could have done. He could have argued that the pardons were illegitimate, were granted in bad faith, and that the president was trying to shield persons from criminal accountability for political reasons and indeed may himself have been involved in a conspiracy to thwart the federal investigation. And presumably if Walsh lost he could have appealed to a higher court, even the U.S. Supreme Court. Whether the courts would have considered his appeal is unclear.
But the history of the pardon power suggests that Walsh may have a plausible argument. Indeed, the fact that the pardon power has never been seriously challenged as being corruptly exercised is curious.