If President Donald Trump pardons Joe Arpaio, as he broadly hinted at during a rally Tuesday in Arizona, it would not be an ordinary exercise of the power -- it would be an impeachable offense. Arpaio, the former sheriff of Arizona’s Maricopa County, was convicted of criminal contempt of court for ignoring the federal judge’s order that he follow the U.S. Constitution in doing his job. For Trump to pardon him would be an assault on the federal judiciary, the Constitution and the rule of law itself.

To see why pardoning Arpaio would be so exceptional -- and so bad -- you have to start with the sheriff’s crime. Arpaio wasn’t convicted by a jury after a trial for violating some specific federal statute. Rather, he was convicted by a federal judge on the rather unusual charge of criminal contempt of court.

Specifically, Arpaio was convicted this July by Judge Susan Bolton of willfully and intentionally violating an order issued to him in 2011 by a different federal judge, G. Murray Snow.

The order arose out of a civil suit against Arpaio brought by the American Civil Liberties Union, accusing him of violating the law by detaining undocumented immigrants simply for lacking legal status.

Snow issued a preliminary injunction that ordered Arpaio to stop running so-called saturation patrols -- police sweeps that essentially stopped people who looked Latino and detained those who were deemed undocumented. The basic idea was that the profiling, warrantless stops and detention were unconstitutional.

Despite the federal court’s order, Arpaio kept running the unlawful patrols for at least 18 months, and publicly acknowledged as much.

Federal judges don’t much like it when their orders are flouted. Snow held extensive hearings in November 2015, and in July 2016 he issued a lengthy opinion finding Arpaio in civil contempt of court. Snow didn’t mince words. He wrote that the department’s “constitutional violations are broad in scope, involve its highest ranking command staff, and flow into its management of internal affairs investigations.”

Crucially, Snow found that Arpaio’s violations had been intentional. But a civil finding of intentionally violating a court order can also trigger a separate proceeding for criminal contempt of court. That’s what happened to Arpaio. To ensure that the judge whose orders were flouted wouldn’t be judging Arpaio criminally, the criminal contempt charges went to a different federal judge.

Bolton convicted Arpaio of criminal contempt. She found he had “willfully violated” the federal court’s order “by failing to do anything to ensure his subordinates’ compliance and by directing them to continue to detain persons for whom no criminal charges could be filed.” And she held that Arpaio had “announced to the world and to his subordinates that he was going to continue business as usual no matter who said otherwise.”

This is the crime that Trump is suggesting he might pardon: willful defiance of a federal judge’s lawful order to enforce the Constitution.

It’s one thing to pardon a criminal out of a sense of mercy or on the belief that he has paid his debt to society.

It’s trickier when the president pardons someone who violated the law in pursuit of governmental policy, the way George H.W. Bush pardoned the Iran-Contra participants, including Oliver North.

But it would be an altogether different matter if Trump pardoned Arpaio for willfully refusing to follow the Constitution and violating the rights of people inside the U.S.

Such a pardon would reflect outright contempt for the judiciary, which convicted Arpaio for his resistance to its authority. Trump has questioned judges’ motives and decisions, but this would be a further, more radical step in his attack on the independent constitutional authority of Article III judges.

An Arpaio pardon would express presidential contempt for the Constitution. Arpaio didn’t just violate a law passed by Congress. His actions defied the Constitution itself, the bedrock of the entire system of government. For Trump to say that this violation is excusable would threaten the very structure on which is right to pardon is based.

Fundamentally, pardoning Arpaio would also undermine the rule of law itself.

The only way the legal system can operate is if law enforcement officials do what the courts tell them. Judges don’t carry guns or enforce their own orders. That’s the job of law enforcement.

In the end, the only legally binding check on law enforcement is the authority of the judiciary to say what the law is -- and be listened to by the cops on the streets.

When a sheriff ignores the courts, he becomes a law unto himself. The courts’ only available recourse is to sanction the sheriff. If the president blocks the courts from making the sheriff follow the law, then the president is breaking the basic structure of the legal order.

From this analysis it follows directly that pardoning Arpaio would be a wrongful act under the Constitution. There would be no immediate constitutional crisis because, legally speaking, Trump has the power to issue the pardon.

But the pardon would trigger a different sort of crisis: a crisis in enforcement of the rule of law.

The Constitution isn’t perfect. It offers only one remedy for a president who abuses the pardon power to break the system itself. That remedy is impeachment.

James Madison noted at the Virginia ratifying convention that abuse of the pardon power could be grounds for impeachment. He was correct then — and it’s still true now.

- Feldman is a Bloomberg View columnist. He is a professor of constitutional and international law at Harvard University and was a clerk to U.S. Supreme Court Justice David Souter. His seven books include “The Three Lives of James Madison: Genius, Partisan, President” and “Cool War: The Future of Global Competition.”

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