During the election campaign and in the first eight months of his presidency, Mr. Trump has done a variety of things that, once fully investigated, may prove to be impeachable offenses.  But we don’t yet know all the facts.  Thus, final judgment on those behaviors will have to wait.

Yesterday, by pardoning former Arizona sheriff Joe Arpaio, Mr. Trump committed his first verifiable impeachable offense.  To some readers, this may seem to be frantic hyperbole.  After all, Article II, Section 2, of the constitution gives a president effectively unlimited to power to pardon anyone (possibly even including himself) for any federal crime except impeachable offenses.

Professor Martin Redish argues in the New York Times that this particular pardon ought to be invalid as a violation of the due process clause, because the only effective redress for those whose rights were violated by then-Sheriff Arpaio when he defied the court’s injunction is a contempt sanction, and voiding that sanction both neuters the judicial power to enforce constitutional rights and deprives Arpaio’s victims of relief.  But even Professor Redish admits that this is a novel theory, and with respect, it simply won’t fly. Any time a president pardons someone for a criminal offense, he voids the considered judgment of some court and deprives the defendant’s victim of redress.  The constitution provides no carve-outs for cases where the underlying crime is a deprivation of civil liberties.

That being so, Trump’s defenders will ask how a president can be impeached merely for exercising a power he undeniably possesses? But this question turns the constitutional function of impeachment on its head.

The founders included in the constitution a congressional power to impeach presidents primarily to respond to misuse by the president of express or implied powers given him elsewhere in the document.

It is true that presidents and other officials can be impeached for conduct not involving the exercise of a specific official power if it intrudes somehow into the sphere of public duty. And impeachment can be proper in the case of a heinous private criminal offense which so far undercuts the moral authority and personal credibility of the offender that he can no longer effectively perform his office.

But, to the founders, the main point of impeachment was that there must be a remedy when a president perverts the powers of his office, either for personal or political self-aggrandizement or, regardless of motive, when the president’s acts threaten the proper distribution of authority among the coordinate branches or otherwise offend either law or fundamental governing norms.

The pardon of Arpaio plainly falls within this core conception of properly impeachable offenses.

• It is an impeachable offense precisely because it involves the exercise of a constitutionally created presidential power.
• The use of the pardon power in this case is a direct assault on core constitutional rights, statutory civil rights laws of the United States, and on the authority of courts to enforce those laws.
• It therefore threatens constitutional civil liberties generally, as well as the viability of congressionally authorized statutory law, and it is a direct attack on the constitutional powers of the judiciary as a coordinate branch of government.
• Accordingly, this pardon threatens to undercut one of indispensable foundational norms of American constitutional order — the rule of law.

One could, of course, make some version of the foregoing argument about many presidential pardons.  Every pardon undercuts a prior judicial decision and vitiates a court’s judgment that the defendant violated a criminal statute and ought to be punished. But here, as elsewhere in the impeachment realm, context and motive matter.

In deciding whether this pardon is impeachable, it matters that its effect (and patent purpose) is to devalue constitutional and statutory protections of a vulnerable minority.  It matters that its effect (and rationally inferable purpose) is precisely to undercut the power of the judiciary to enforce the law against officials who believe they can violate it with impunity.  And it matters that Mr. Trump’s motive in issuing the pardon is so transparently political. This is not a considered judgment that a particular individual has been unfairly treated by corrupt judges, a flawed process, or an unjust law.  It is, instead, a transparent pander to a politician’s political base.

The fact that the constitution grants the president the theoretical power to behave in this way does not deprive Congress of the power to conclude that the exercise of the power is so contrary to constitutional principles and democratic norms that it constitutes an impeachable offense.

As always, sober pragmatism reminds us that no presidential behavior, however egregious, will result in impeachment and removal unless a majority of the House and supermajority of the Senate deem it so, and that the Republicans controlling both houses have so far shown no disposition to take these steps.  The sole point here is that, should that political obstacle ever dissolve, there is now at least one constitutionally sound basis upon which impeachment could be based.