Except it has happened before. It was well over a century ago, but it has happened. And that federal court didn't condemn such a request by the president as criminal or impeachable; in fact, the request was not even worthy of criticism.
In an 1878 case brought before a grand jury in the District of Indiana, a prosecutor received a request from President Rutherford B. Hayes not to prosecute a citizen for embezzlement from a national bank. The case, In Re Miller (short for "In the Matter of the Interference of the President with the Prosecution of Casey Miller before the Grand Jury"), didn't flesh out the extent of the president's power to order executive branch officials to refrain from prosecution. But the court did observe
that the president "may, if he feels so inclined, interfere, even in advance of indictment, by exercising the pardoning power."
Significantly, the Miller court also gave instructions to a sitting grand jury -- not a prosecutor or law enforcement official. The court advised the grand jury that the president does not have the "slightest authority to control" its action in conducting an investigation. Why? Because the grand jury is part of the judicial branch, and the president doesn't have direct authority over this branch. In contrast, prosecutors and investigators, like the FBI, are part of the executive branch and serve under the president.
The Miller case is admittedly about the clearer principle that the head of the executive branch cannot control or even encroach upon the action of the courts in the administration of criminal justice. In the unintentionally prescient words of the 19th century Miller court, the president has "no more right to control [the judicial branch] than the czar of Russia."
But most importantly, the case is an example of a federal court recognizing -- and even accepting -- that the president may occasionally try to interfere with federal investigations or prosecutions. The judicial response to this kind of executive intervention is apparently not a cry for impeachment or prosecution. Instead, the remedy appears to be no more than an instruction to the members of a grand jury to, well, just ignore the president when he does that.
In modern times, some courts have indicated that an obstruction case requires more than just an attempt to influence official proceedings. One learned federal judge, the Honorable Abner J. Mikva, even suggested
in an opinion that "executive personnel constantly attempt, in innumerable ways, to obstruct or impede congressional committees as part of legitimate political jousting between the executive and legislative branches."
Concededly, Judge Mikva's observations were contained in a non-binding dissent, in a decision that was later obviated when Congress redefined "corruptly" in subsequent legislation. But the point is that persons within the executive branch may be allowed to engage in a little advocacy and even manipulation of congressional investigations. It follows then that the chief of that branch might enjoy at least a commensurate -- if not superior -- privilege to try to influence investigations within his own branch. It's a fine line, though. Once one attempts to persuade, and does so "corruptly," the activity violates any of a dozen different obstruction and related federal crimes.
The problem is that even the federal appellate courts have disagreed in defining what it is to "corruptly persuade" in certain kinds of obstruction cases.
In fact, the courts only seem to agree on one thing in defining "corruptly": the phrase itself is ambiguous
. Some federal appellate courts have taken the view that any persuasion with an "improper purpose" qualifies. That would include just one's own self-interest in seeking to impede an investigation. Other federal appellate courts have required something
more inherently wrongful about the persuasion, like encouraging someone to testify falsely.
Like a lot of federal crimes, the statutes criminalizing obstruction of justice are so broadly drafted, reasonable minds can, and have, differed on what conduct is prohibited.
Could it be that the president can never obstruct justice? No, that's probably not the case. In fact, it seems pretty clear that when anyone uses, say, deception or intimidation
to prevent the production of evidence, then that person obstructs justice.
But merely suggesting to an FBI official that he should drop an investigation? If not done "corruptly" -- whatever that means -- it might be okay. At least, it was apparently okay back in 1878, when there was still a czar of Russia.