THE FIRST debate between President Trump and Democratic challenger Joe Biden, on Tuesday night at Cleveland’s Case Western Reserve University, was a disgrace. But it was not without value.

Voters learned that the president cannot bring himself to condemn white-supremicist groups or admit that human activity is the primary driver of climate change or unambiguously tell people to wear masks. Americans learned that Mr. Trump’s health-care plans crumble at even the slightest scrutiny. People learned that he believes he has done a “phenomenal” job on the novel coronavirus and that the country should not expect any better from him on protecting public health.

The nation learned — or relearned — that Mr. Trump has no positive case for reelection, let alone a vision of where to take the country in the next four years. When he was not hurling insults at Mr. Biden, he tried to take credit for an economy he did not build or to allege that Mr. Biden would destroy the suburbs.

And voters learned — and learned and learned and learned — that Mr. Trump has nothing but contempt for the values and norms that are essential to democracy: among them, truth, civility and respectful disagreement. Mr. Trump’s lies about mail-in ballots were poisonous, and his attacks on Mr. Biden’s family were particularly vile. But the lowest point came when the president would not commit to accepting legitimate election results or to tell his supporters to refrain from causing civil unrest. Instead, he said that he is “counting on” a Supreme Court with a new Trump-appointed justice to “look at the ballots.” He called on his supporters to swamp polling places to look for “bad things,” a recipe for chaos. The violent hate group Proud Boys, he said, should “stand back and stand by.”

Taken together, Mr. Trump’s message and behavior reflected a deep contempt for the nation and its voters. His actions presaged an attempt to reject and delegitimize the election results, while inciting violence. That’s a threat that must be taken seriously by election and law enforcement authorities — and by responsible leaders in both parties.

About Mr. Biden, Americans learned that the former vice president refuses to surrender to the left wing of his party, as Mr. Trump so often alleges he has. Instead, Mr. Biden outlined an ambitious yet moderate approach to improving health care and addressing climate change. Americans saw that Mr. Biden will not promise miracle cures for covid-19 or promote a precipitous reopening that only spreads the virus and forces more lockdowns. And the country learned that Mr. Biden sticks to his message of calm and unity even under intense personal attacks.

Of course, Americans would have learned a lot more if Mr. Trump had obeyed the rules he agreed to before the debate, allowing Mr. Biden to answer questions without constant interruption. It would have been more edifying if the president had brought more than falsehoods, personal attacks and threats to the stage. The Commission on Presidential Debates announced Wednesday that “additional structure should be added to the format of the remaining debates to ensure a more orderly discussion of the issues.” We agree: If there are to be two more debates, the moderators should be given control of the candidates’ microphones — and they should cut off the president if he again shamelessly disregards every principle of decorum and decency.

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