Then-Republican presidential candidate Donald Trump is joined by Joe Arpaio. (Mary Altaffer/Associated Press)

Laurence H. Tribe is university professor and professor of constitutional law at Harvard Law School. Ron Fein is the legal director of Free Speech for People, which has filed an amicus brief in the Arpaio case.

A federal judge in Arizona will soon consider whether to overturn President Trump’s pardon of former Arizona sheriff Joe Arpaio. The answer to this question has consequences not just for Arpaio and the people he hurt but also for the entire country. And although the conventional legal wisdom has been that a presidential decision to grant a pardon is unreviewable, that is wrong. In this circumstance, Trump’s decision to pardon Arpaio was unconstitutional and should be overturned.

For more than 20 years, Arpaio ran the Maricopa County Sheriff's Office with shocking cruelty and lawlessness, especially against Latinos. In 2011, a federal judge issued an injunction in a lawsuit challenging the practice of detaining and searching people for, in essence, driving while Latino. The judge found evidence that the sheriff's office engaged in racial profiling and stopped Latinos just to determine their immigration status. He ordered it to cease detaining people without reasonable suspicion of a crime.

Arpaio flagrantly ignored the injunction, and in May 2016, the judge found him to be in civil contempt of court. In July, a second federal judge found him in criminal contempt, which can be punished by imprisonment. Three weeks later, Trump pardoned Arpaio. Trump's Justice Department argues that is the end of the matter, but many constitutional law scholars and advocates disagree. The judge has scheduled an Oct. 4 hearing and ordered further briefing.

To understand why Trump's pardon of Arpaio is so dangerous, step back to 1962, when a federal court ordered the all-white University of Mississippi to admit African American James Meredith. When the Mississippi governor refused to comply, the court directed the Justice Department to prosecute him for criminal contempt of court.

At the time, many anti-integration governors vowed "massive resistance" to court-ordered desegregation. The legal struggle against segregation relied on the power of court orders — enforceable by imprisonment for contempt.

Now imagine a president such as Trump pardoning the governor for contempt, while praising him, as Trump lauded Arpaio, for "doing his job."

The message to segregationist officials would have been clear: just ignore federal court integration orders; the president will have your back if the court tries to enforce them through its contempt power.

No official could have failed to hear that message in the Arpaio pardon. Nor could have any Trump associate in possession of evidence bearing on criminal or impeachable activity by the president. The signal to them all is: You can treat any court order to testify, even with a grant of immunity, as a friendly request and politely decline. You can count on a pardon to protect you from criminal prosecution.

Is this use of the pardon power constitutional? In most cases, however controversial, courts should not second-guess the president's use of the pardon power. But when the Constitution says that the president "shall have Power," that does not mean unlimited power. It means power that is not inconsistent with other parts of the Constitution.

For example, the Constitution says "Congress shall have Power To lay and collect Taxes," but that doesn't mean Congress can tax white people at a different rate than black people. The Constitution says the president "shall have Power" to make treaties, but that doesn't mean he can make a treaty that abolishes freedom of speech.

The power to pardon is limited by the Fifth Amendment's guarantee that no person be "deprived of life, liberty, or property, without due process of law." That guarantee requires that courts must be able to issue and enforce injunctions to stop constitutional violations by government officials. Otherwise, compliance with a court order would be optional.

In 1925, the Supreme Court upheld President Calvin Coolidge's pardon of a speakeasy operator who had violated a court order to stop selling liquor during Prohibition. But the court has not considered a pardon for contempt since 1925. And it has never considered a pardon issued to a ranking government official for disobeying a court order to stop a systemic practice of violating individuals' constitutional rights.

The framers suggested one solution to the prospect of such abuse. During a Virginia debate over whether to ratify the Constitution, George Mason worried that the president might "pardon crimes which were advised by himself." James Madison replied that a president who did so could be impeached. Trump's pardon of Arpaio should trigger congressional hearings on whether it constitutes an impeachable offense.

But it strains logic to suggest that, although a president can be removed from office for an unconstitutional pardon, the pardon itself must be judicially enforced. By pardoning Arpaio for his willful disobedience of a court order to stop violating Arizonans’ constitutional rights, Trump has pulled the republic into uncharted waters. Our best guide home is the Constitution.