President Trump at the White House. (Jabin Botsford/The Washington Post)

In a now-infamous Saturday tweet, President Trump appeared to admit he knew former national security adviser Michael Flynn had lied to the FBI when Trump allegedly pressured then-FBI Director James B. Comey to drop the Flynn investigation. The White House response to this new evidence of potential obstruction of justice was first to claim the tweet was composed not by Trump but by his lawyer, John Dowd. Dowd later asserted that, regardless of whether or why Trump pressured Comey, the president could not be charged with obstruction of justice. Dowd claimed that as the country's chief law enforcement officer, the president "has every right to express his view of any case" and such conduct could not amount to obstruction.

Unfortunately for Trump, there's no sound basis for this claim of presidential immunity.

Dowd's argument echoes a claim made for months by Harvard Law School professor emeritus Alan Dershowitz. Dershowitz argues that a president cannot be charged with obstruction of justice for engaging in "constitutionally authorized acts." As the head of the executive branch, Dershowitz argues, Trump has the absolute right to fire the FBI director and to decide which cases will be pursued. To charge a president with obstruction based solely on such actions, he claims, would cause a "constitutional crisis."

In Dershowitz's view, only if the president also engages in some other criminal conduct would an obstruction of justice charge be justified. "Nixon," Dershowitz says, "told aides to lie to FBI, pay hush money and destroy evidence," and so obstruction charges were appropriate in that case. But if all Trump did was pressure or fire Comey, those acts are not independently unlawful and could not be obstruction. If Flynn told the president, "The FBI is squeezing me and I may have to rat you out on this whole Russia thing," and Trump subsequently ordered Comey to drop the case, Dershowitz argues that would be perfectly legal.

It's true the president has the right to weigh in on a case or fire the FBI director, but he does not have the right to do so for corrupt purposes. I have the right to shred my personal files, but if I destroy them because they've been subpoenaed by the grand jury it becomes obstruction. There have been many cases where individuals engaged in otherwise lawful actions have been charged with obstruction of justice for taking those actions with corrupt intent.

Dowd and Dershowitz's argument — that Trump pressuring Comey to drop the investigation could not be obstruction no matter how corrupt the motive — has some chilling implications. It suggests a president would never need to bother with destroying evidence, tampering with witnesses or other directly obstructive conduct. The president could simply order any investigation into his own potential criminal wrongdoing to be shut down, because that is a "constitutionally authorized" act.

Under this theory, if President Richard Nixon had simply fired Archibald Cox and ordered a halt to all investigations, he would have been immune from prosecution. If Dershowitz is correct, Trump is wasting everyone's time. He should just order that special counsel Robert S. Mueller III's investigation cease immediately, claim that his actions are "constitutionally authorized" and move on.

Although it's unclear, Dershowitz may actually agree that impeachment would be appropriate in such a case. Sometimes he argues that impeachment could be justified, but other times he appears to claim it would be unconstitutional for Congress to even charge the president with obstruction absent additional criminal conduct. His analogy to Nixon, who was threatened with impeachment but not prosecuted, suggests he thinks not even Congress could proceed against Trump in these circumstances. And if he believes Trump's actions were "constitutionally authorized," it's hard to see how he could also believe the Constitution authorizes his removal from office for those actions. But even assuming impeachment remains an option, that does not explain why the criminal laws concerning obstruction of justice would not apply to the president. Impeachment results only in removal from office, and the Constitution expressly provides that an impeached official remains subject to indictment, prosecution and punishment for any criminal violations.

Certainly proving obstruction by the president should be a high bar, and it's not clear that Mueller will find sufficient evidence of obstruction by Trump. If he does, it's not clear that a sitting president can be indicted and prosecuted. Even if he can, it's not clear that Mueller would take that step, rather than referring the matter to Congress for potential impeachment proceedings. Impeachment may in fact be the best and most appropriate remedy if obstruction of justice is found.

But what is clear is that Dershowitz and Dowd have cited no legal authority for the extraordinary proposition that, when it comes to obstruction, the president is above the law. Nixon famously told David Frost, "When the president does it, that means it is not illegal." Dershowitz and Dowd are making a variation of that same argument — but pursuing that particular constitutional theory did not end well for Nixon.