If President Donald Trump asked Federal Bureau of Investigation (FBI) director James Comey to stop investigating National Security Adviser Mike Flynn and his ties to Russia, that is obstruction of justice.
But let us be clear: It is the impeachable offence of obstruction.
It is probably not the criminal version of that act. With the evidence now available, it is extremely unlikely that an ordinary prosecutor could convict Mr Trump.
This is an outstanding example of a crucial distinction that Americans badly need to keep in mind. High crimes and misdemeanours - to use the Constitution's phrase - are not the same as ordinary crimes.
What makes them "high" is their political character. High crimes and misdemeanours are corruption, abuse of power and undermining the rule of law and democracy.
They do not have to satisfy all the technical aspects of an ordinary crime. And this act of Mr Trump's, as described in a memo written by Mr Comey that was first reported on Tuesday by the New York Times, probably does not.
Start with the federal obstruction statute, 18 USC Section 1503.
The first part of the law has to do with trying to influence jurors in the course of a trial; we can ignore it for our purposes.
The second part of the law punishes anyone who "corruptly or by threats or force, or by any threatening letter or communication, influences, obstructs, or impedes, or endeavours to influence, obstruct, or impede, the due administration of justice". On a close reading, this is not a great fit with the President asking the FBI director if he can let a probe go because the target is "a good guy".
As a constitutional matter, the FBI director, like the attorney-general and the rest of the machine of federal law enforcement, works for the president. Although there has been a strong tradition of separating investigation and prosecution from the president - a tradition grossly violated by Mr Trump's request - it is still just a tradition, not a legal requirement.
Thus, as a constitutional matter, Mr Trump has the authority to propose ending a probe. If he wanted to, Mr Trump could just order the investigation to be brought to an end. He would not even have to exercise his pardon power, another way to put a pre-emptive stop to investigations.
He could just direct his subordinates to cease. To be sure, Mr Comey probably would have resigned had this order been given.
The point is that Mr Trump could have given it, legally speaking.
Given Mr Trump's inherent constitutional authority to end the Flynn investigation himself, it is pretty hard to say that he was "corruptly" obstructing or impeding "the due administration of justice". It is within prosecutorial discretion to decide not to go after someone because he is a good guy.
The target's character and career of public service are legitimate factors to consider in the course of the investigation and decision of whether to bring charges.
It is not that the president can never be guilty of the crime of obstruction. He can. It would be a federal obstruction crime for the president to lie to or to mislead investigators. It would be an obstruction crime for the president to hide evidence of a crime.
But those examples are fairly different from the president exercising authority over investigations.
The one credible legal argument that could be made by a prosecutor seeking to charge Mr Trump would be that he was acting "corruptly" if his true intent was to protect himself and his administration, not just give Mr Flynn a break.
Suppose a president owed a favour to an organised crime leader and asked the FBI director to drop the investigation. That would presumably count as a corrupt act, and would count as obstruction.
It is not at all clear how you could prove Mr Trump's intent here, except maybe by taped conversations where he says he wants to protect Mr Flynn to protect himself. Nor is it at all clear that acting "corruptly" under the statute would include saving himself from embarrassment. The upshot is that I do not think Mr Trump could be prosecuted for a crime on the basis of this report, and I am not at all sure that he actually committed a federal crime, legally speaking.
Impeachment is another matter. Using the presidential office to try to shut down the investigation of a senior executive official who was also a major player in the President's campaign is an obvious and egregious abuse of power. It is also a gross example of undermining the rule of law.
This act is exactly the kind that the Founding Fathers would have considered a "high crime". And it is a high crime the president could perform only by virtue of holding his office.
It still seems unlikely that a Republican House would impeach the President, much less that two-thirds of the Senate would vote to convict and remove him from office. But a Democratic House would have more than enough material now to start the impeachment process - including the revelation of the request to Mr Comey. And the House could choose to impeach even if it calculated that the Senate probably would not convict.
The act of impeachment would have tremendous symbolic ramifications, and it would include the detailed investigative oversight that is lacking in Washington.
Mr Trump's firing of Mr Comey now looks pretty different in the light of this news. Right around now, the President is probably asking himself whether firing the FBI director was the right decision. And if he is not, he should be.
•The writer is a professor of constitutional and international law at Harvard University. His seven books include The Three Lives Of James Madison: Genius, Partisan, President.