Robert Bauer, professor of practice at New York University School of Law, served as White House counsel to President Barack Obama.

Since January, examples have piled up of a pattern of recklessness, impropriety and perhaps outright obstruction in President Trump’s oversight of federal law enforcement. And now, with the pardoning of Joe Arpaio, we have the first exercise of that power in a different context, perhaps serving for Trump as a test run for shutting down the investigation into ties between his campaign and Russia. No doubt the president is acting on his belief that, because the pardon power is “complete,” it cannot cause him real harm, and all the rest is politics. This is a miscalculation.

The instances of Trump’s warped approach to the law are legion: the demeaning of his attorney general for recusing himself, as required by the Justice Department rules, in the Russia matter; the demand for personal loyalty from James B. Comey and the request that the then-FBI director desist from investigating the conduct of his former national security adviser; the appeal to the heads of intelligence agencies to help him contain Comey; the firing of Comey for his continued pursuit of the Russia case; and the threats to fire special counsel Robert S. Mueller III. Trump has since repeatedly tweeted his denunciation of that investigation — a major, ongoing federal criminal inquiry — as a “hoax.” He has hinted in clear terms that he could end all of these troubles by the use of the “complete” pardon power.

Trump’s record on “rule of law” issues, now including this pardon, weakens his defenses in the Mueller probe — and in any future debate over impeachment. Instead, for a president contending with questions of obstruction of justice in the Russia matter, the pardon casts his modus operandi in the worst possible light. The former sheriff was convicted in July of criminal contempt , and Arpaio’s attorney announced that he would appeal. Only weeks later, Trump highlighted a possible pardon at a political rally. He impugned the seriousness and motive of the court’s judgment, dismissing it as an attack on Arpaio for “doing his job.” Days later, without Justice Department review, he extinguished the conviction and ended the case.

There is a line that distinguishes a pardon from direct interference with the administration of justice. Trump has crossed it. The difference between this form of interference and the others is, simply, that the use of a pardon has enabled Trump to engage in obstruction unilaterally, not needing a willing partner in the Justice Department. And it is now reported that Trump did, in fact, first try to have the department drop the case against Arpaio.

The statement the White House issued is hopelessly inadequate in justifying the pardon. The true rationale remains a subject for speculation. But there are clues, and they all point in the same direction: pure political self-interest. The president touted the pardon at a political rally when he encouraged a sympathetic crowd of supporters to register loudly how much they “like” Arpaio, a political ally whom the president also appears to “like” personally. The timing of the pardon suggests that Trump’s politics may have entered into the decision from another direction: a bid to soften the blows now raining down on him from the Breitbart wing of the party in the aftermath of Stephen K. Bannon’s departure.

So the president has situated this pardon squarely within the realm of politics, not criminal justice. The action was not consistent with the constitutional norm that pardons are a judiciously considered act of grace, a measure to correct for injustice, or otherwise related to the president’s constitutional responsibility for the public welfare. It exposes, as do other Trump’s other interventions in law enforcement, the president’s blindness to the difference between his own interests and his obligations to the constitutional duties, values and norms he took an oath to defend, including the “faithful” execution of the laws. This supremacy of self-interest is also evident in his refusal to separate himself from his business interests and his continuing promotion of those interests by ostentatiously arranging events on properties from which he derives personal income.

Of course, all presidents must and do, in some cases and to some degree, weigh politics in the balance in meeting their responsibilities. But Trump shows no sign of knowing when political considerations are appropriate, and he does not do “balance.” This is all, it is increasingly clear, beyond him.

Trump seems to believe that the pardon power is so “complete” that it is his ace in the hole, his ultimate protection. But it will be of no use to him if the time comes — after further developments in the Russia case, or for other reasons in this deeply troubled administration — that Congress must consider impeaching him for systematically violating his oath to put his constitutional obligations ahead of his political or personal interests. At that time, the Arpaio pardon is sure to be part of the story of this presidency and, very conceivably, of how it came to an end.