Even the Liberal Alan Dershowitz is spouting a bunch of nonsense about how, since the POTUS has the legal authority to fire the director of the FBI, exercising that authority can’t be obstruction of justice by definition.
This makes about as much sense as arguing that since the president has the legal authority to command the armed forces, ordering the army to kill his political opponents can’t be murder.
Now the intersection between the criminal justice system and the office of the president is a murky area in the law. For example, t’s not clear whether a sitting president can be indicted for a crime while he’s in office, or if he can pardon himself. (It would be nice to think that there’s currently no formal resolution of these issues because the people who formulated these rules were innocent enough not to consider the possibility that anybody could be shameless enough to pardon himself). It’s certainly possible that the federal courts would hold that the POTUS can’t be prosecuted for obstruction of justice while still in office. But, as a practical matter, there isn’t the slightest doubt that Congress has an essentially un-reviewable power to decide what constitutes obstruction of justice for the purposes of impeachment and removal.
In short, the claim that the president can’t obstruct justice by doing an otherwise legal act is, if not quite frivolous in the formal legal sense, an obviously specious argument. It’s the kind of argument that should get somebody the lowest grade they give out these days at Dershowitz’s law school, which I understand is something like an A almost minus. To present it as a self-evident truth is either the epitome of bad faith, or a product of McCain-level cognitive deterioration.