President Trump pardoned former Maricopa County, Ariz., sheriff Joe Arpaio Aug. 25. Here's what you need to know. (Patrick Martin,Victoria Walker/The Washington Post)

Last month, as President Trump made broad claims about his power to pardon, I noted that he “may find out that something can be both legal and, simultaneously, an impeachable offense.” Last night, as the president issued a pardon to former Maricopa County, Ariz., sheriff Joe Arpaio, who was convicted of criminal contempt of court, some commentators argued that this was exactly the case.

Harvard Law professor Noah Feldman, for example, wrote after Trump’s belligerent Phoenix rally speech that such a pardon would represent an “assault on the federal judiciary, the Constitution and the rule of law itself” for which the “remedy is impeachment.”

It is hard to gauge the political fallout of the president’s decision — announced as it was late on a Friday night during an impending hurricane. Normally, though, as political scientist Jeffrey Crouch’s book on the pardon power makes clear, pardons are granted for two reasons: either to provide mercy or correct a miscarriage of justice, in an individual case; or on more general grounds based on public policy.

Trump’s pardon of Arpaio does not fit either category very well.

As regards mercy: Alexander Hamilton wrote in the Federalist that pardons were needed; otherwise, “justice would wear a countenance too sanguinary and cruel.” Presidents have sometimes pardoned elderly convicts, for instance, rather than see them die in prison.

Arpaio is 85, but he had not even yet been sentenced; that hearing was set for October. As a procedural matter, the guidelines of the Justice Department’s office of the pardon attorney — not binding on the president, of course, and not consulted in this instance — state that petitions for clemency are normally considered only after five years have passed after a conviction. (Further, in considering such petitions, “The extent to which a petitioner has accepted responsibility for his or her criminal conduct and made restitution to its victims are important considerations.”)

Pardons also serve as a check against the judicial branch, when the president feels a grave miscarriage of justice has occurred. At his Phoenix rally, Trump seemed to make this claim, saying that “Sheriff Joe was convicted for doing his job.”

The problem with that, though, is that Arpaio was convicted for doing the opposite of his job. As a sworn officer of law enforcement, he violated the law and then ignored court orders designed to bring his policies in line with statutory and constitutional mandates. Two different federal judges found, respectively, that the “constitutional violations” committed by Arpaio’s office were “broad in scope, involve its highest ranking command staff, and flow into its management of internal affairs investigations” and that he “willfully violated” directives to correct those violations.

That in turn circles back to the public policy rationale for pardons. Presidents have given clemency to both individuals and groups, arguing that doing so serves the broader public good — such cases range from Thomas Jefferson’s 1801 pardons of those convicted under the Alien and Sedition Acts to Gerald Ford’s 1974 pardon of former president Richard M. Nixon to Barack Obama’s commutation of more than 1,700 prison terms he thought were skewed by the past mandatory imposition of long sentences even for nonviolent crimes.

Here, though, it is hard to see how the public interest has been served. Rather than “restore the tranquility of the commonwealth” (as Hamilton thought a pardon might do), Trump’s action seems likely to harden its divisions. Arpaio’s status as what George W. Bush speechwriter David Frum calls “America’s second most famous Obama birther” and his long history of abusing his office hardly makes him a symbol of the unity the president has intermittently claimed to desire after Charlottesville. And pardoning a sheriff for disobeying federal law is substantively out of step with the constitutional mandate that the president faithfully execute that law — and with the foundational American concept of “a government of laws and not of men.”

Hence the arguments that Trump’s action — though itself clearly legal — undermines the rule of law. Is it an impeachable offense? That will depend, as Gerald Ford put it early in his political career, on whether “a majority of the House of Representatives considers it to be …”

President Trump spoke about possibly pardoning former Maricopa County, Ariz., sheriff Joe Arpaio during a rally in Phoenix on Aug. 22. Arpaio was convicted of criminal contempt in July. (The Washington Post)