CHICAGO — Note: Friday night, after this Op-Ed was published, it was announced that President Trump pardoned Joe Arpaio.

At his rally in Phoenix on Tuesday, President Trump strongly implied that he would pardon Joe Arpaio, the former sheriff of Maricopa County, Ariz., who was found guilty in July of criminal contempt for defying a judge’s order against prolonging traffic patrols targeting immigrants. This is not idle presidential chatter: On Thursday morning, CNN reported that the White House has prepared the necessary paperwork, along with talking points for its allies.

This is uncharted territory. Yes, on its face the Constitution’s pardon power would seem unlimited. And past presidents have used it with varying degrees of wisdom, at times in ways that would seem to clash with the courts’ ability to render justice. But the Arpaio case is different: The sheriff was convicted of violating constitutional rights, in defiance of a court order involving racial profiling. Should the president indicate that he does not think Mr. Arpaio should be punished for that, he would signal that governmental agents who violate judicial injunctions are likely to be pardoned, even though their behavior violated constitutional rights, when their illegal actions are consistent with presidential policies.

Many legal scholars argue that the only possible redress is impeachment — itself a politicized, drawn-out process. But there may be another route. If the pardon is challenged in court, we may discover that there are, in fact, limits to the president’s pardon power after all.