Donald Trump turned, in the third and final presidential debate, from insulting the intelligence of the American voter to insulting American democracy itself. He falsely insisted there were “millions of people” registered to participate in the election who did not have the right to vote and declared he would not commit to honoring the outcome.

Hillary Clinton was clearly shocked that he was attacking the very foundation of the republic, the American tradition of peacefully transferring power. “That’s horrifying,” she said, rightly. At one point, Mr. Trump even said, outrageously, “She shouldn’t be allowed to run.”

The presidential debate was another exercise in narcissism, bombast and mendacity by Mr. Trump. One could only hope that this might be the last grand display of his gross unfitness to be president.

Mr. Trump arrived at the debate in Las Vegas after days of making venomous attacks on the democratic process, and by implication, the voters’ ability to make sound choices. Asked about whether he would accept the election result, he tersely answered, “I will look at it at the time.” In rejecting his answer, Mrs. Clinton noted that Mr. Trump is a chronic complainer when he loses, even in an Emmy award competition. But applying his loser’s lament to an American presidential election is a far different proposition than whining about a TV show. “He is talking down our democracy,” Mrs. Clinton warned.