Republican U.S. presidential nominee Donald Trump. (Carlos Barria/Reuters)

DONALD TRUMP showed a bit more self-control in the third and final presidential debate Wednesday night than he had in the previous two. His back and forth with Hillary Clinton was more substantive, thanks in part to firm guidance from moderator Chris Wallace. But all of that was overshadowed by Mr. Trump’s breathtaking refusal to say that he will accept the results of the election.

“I will look at it at the time,” he said. “The media is so dishonest and so corrupt . . . they poison the minds of the voters . . . She should never have been allowed to run for the presidency.”

Ms. Clinton rightly called his stance a “horrifying” repudiation of U.S. democracy. Respecting the will of the voters has since the end of the Civil War allowed for a peaceful transition of power that has made this country the envy of the world.

Republican presidential nominee Donald Trump said, "I'll keep you in suspense," when answering a question about the tradition of accepting the results of presidential elections. (The Washington Post)

Next to that, policy issues seem small. Yet the policy discussion was clarifying also, exposing as it did Mr. Trump’s ignorance of — or is it distaste for? — facts and policy. He again insisted that the North American Free Trade Agreement has sucked jobs from the country, when economists have found otherwise. He indicated the debt would take care of itself under his economic plan because “we will have created a tremendous economic machine,” which is pure snake oil. Incoherently, he attacked Ms. Clinton for favoring open borders but also favoring a border wall.

In another striking moment, Mr. Trump denied that the Russian government has been meddling in this election, refusing to accept the judgment of the country’s intelligence community. Ms. Clinton said “the most important question” was whether Mr. Trump would acknowledge Moscow’s interference. Mr. Trump at first declined to do so, saying he doubted the reports by U.S. intelligence agencies. He avoided any criticism of Russia’s Vladi­mir Putin, repeatedly insisting it would be “good” to get along with Russia, with no mention of Russia’s invasion of Ukraine and other actions that have made getting along difficult.

As if to prove Ms. Clinton’s point that Mr. Trump would withdraw U.S. leadership from the world — to Mr. Putin’s delight — the Republican nominee doubled down on his insistence that NATO countries and other allies “have to pay up,” and he absurdly took credit for forcing reforms on the decades-old alliance. Ms. Clinton, by contrast, insisted that the United States benefits from engagement in the world, which used to be a consensus view on presidential debate stages.

When Mr. Wallace turned to the scandals that have dominated the past month, Mr. Trump incorrectly insisted that the women who have come forward to accuse him of sexual misconduct have been “debunked.”

Ms. Clinton managed to dodge some questions, including on communications that took place between the Clinton Foundation and the State Department under her leadership. She rightly said that she would not worsen the national debt as Mr. Trump would, but she could not refute Mr. Wallace’s point that she has no plan to reduce it. She had no clear answer on how she could impose a no-fly zone over Syrian airspace now controlled by Russia.

These are gaps that would have been probed and tested in a normal campaign. They fade to the status of trivia in the face of an opponent who will not accept the basic rules of American democracy.