The president can pardon virtually anyone he wants, which makes it more telling that he chose to wield the power for the first time in favor of Mr. Arpaio, an officer of the law who defied a court order. It shows not only contempt for the judiciary’s sole means of enforcing the law, but suggests that Mr. Trump may be just as eager to pardon friends, family and allies caught up in the Russia investigation.

The Arpaio pardon is not only morally reprehensible on its own, it is also in line with Mr. Trump’s broader attitude toward law enforcement. Consider his affection for the Milwaukee County sheriff, David Clarke, an Arpaio in waiting who has called activists in the Black Lives Matter movement “terrorists” and who runs a county jail where inmates have a tendency to die under suspicious circumstances. (On Sunday morning, as Hurricane Harvey raged across Texas, Mr. Trump tweeted out a plug for Mr. Clarke’s new book and called him a “great guy.”)

The pattern goes back much further, as The Times’s Maggie Haberman wrote on Monday. During the presidential campaign, Mr. Trump endorsed the use of torture on terrorism suspects, encouraged supporters at his rallies to assault protesters and made racially tinged comments about a judge overseeing a case involving Trump University.

In his seven months as president, Mr. Trump has attacked federal judges who ruled against the administration’s travel ban; tried to impede investigations into his allies, including Mr. Arpaio; and exhorted police officers to treat suspects roughly — which earned a quick rebuke from his own Justice Department and police officials around the country.

Rebukes, from his advisers and members of Congress, grow more frequent. Gary Cohn, director of the White House Economic Council, nearly resigned after Mr. Trump’s Charlottesville remarks. House Speaker Paul Ryan said he opposed the Arpaio pardon, and Senator John McCain said it undermined Mr. Trump’s “claim for the respect of rule of law.”

But this is Donald Trump’s rule of law — a display of personal dominance disconnected from concerns about law and order, equality or the Constitution. That distorted understanding of justice is cleaving the nation between the majority who support the principles of American democracy and those who support only him.