Immigration and workers' rights advocates protest outside the site of the Republican Presidential Debate in Simi Valley, Calif., on Sept. 16. (Robyn Beck/Agence France-Presse via Getty Images)

OVER THE past weeks we have used some sharp words in our editorials about the race for the Republican nomination — words such as bigot, bully and buffoon. Some readers have asked whether by so doing we undermine our own calls for civil discourse. The answer has a lot to do with this moment in American history — a dangerous moment when something ugly is taking place in the political arena. It’s a time that demands a sharp and clear response from everyone who cares about fairness and decency, democracy and tolerance.

Generally the system works best when people assume that their political opponents are acting in good faith. We may feel strongly about gun laws, campaign finance or free trade, but we recognize that there are defensible arguments on the other side. In the heat of the debate, we sometimes fall short of our aspirations, but as U.S. politics become ever more partisan, it becomes ever more important to give opposing views a fair hearing. That’s one reason we publish a range of opinions on the facing page, especially ones that differ from our own.

But Donald Trump and his imitators present a different kind of challenge to democratic discourse, in at least three ways. Mr. Trump, the leading candidate for the Republican nomination, seeks to make his political fortune not by staking out and defending positions but by fanning and exploiting hatred and fear. He says and repeats things that are demonstrably false, which makes a mockery of legitimate debate. He prefers to insult, demean and ridicule anyone who challenges him rather than to engage meaningfully with their arguments.

The essence of his campaign has been to portray those who are different from him and his supporters as unworthy, less than human and so deserving of abuse. His incendiary language associates Mexicans with rapists and Muslims with terrorists. The demonization then is used to justify the unjustifiable: mass deportations for undocumented immigrants, torture for suspected terrorists, bombing enemies’ innocent relatives, barring all Muslims, beating up an African American protester.

Sen. Ted Cruz (R-Tex.) follows a similar playbook when he asserts, falsely, that immigration reform aimed at bringing undocumented immigrants out of the shadows would have given President Obama the authority to admit “ISIS terrorists.” Even his ostensibly humorous reference to “undocumented Democrats” serves to dehumanize. The Salvadoran woman worrying whether her children have done their homework as she works the night shift at a fast-food restaurant is no longer a person trying to give her kids a better life but a political token, deserving of no sympathy. It is legitimate to debate the proper level of immigration, but that’s not Mr. Cruz’s goal. When he echoes the segregationist Alabama governor George Wallace in his denunciation of a path to legalization, he is making a very different kind of argument.

We are told that Mr. Trump’s ugliness has found an audience because Americans are resentful and afraid. Globalization and technological change have left many behind. Rapid immigration has made some feel like strangers in their own country. After the shootings in Paris and San Bernardino, Calif., some wonder whether their government can keep them safe. Each of these issues could and should be addressed in a presidential campaign. It is Mr. Trump’s noxious choice to conflate all three without offering real solutions to any of them.

What is the right way to respond, and who is the right audience? Mr. Trump believes in nothing other than his own aggrandizement. Mr. Cruz is not going to let principle or conviction, if he has any, stand between him and his ambition. They are not the readers we have in mind when we write. It would be nice if Republican leaders who know the Trump message is un-American but have been too timid to speak out would rise to this occasion, but it seems we can’t count on that, either.

No, the audience that matters at this moment is all of the rest of us — Americans who know that our country is better than Mr. Trump and Mr. Cruz understand. Our history can be read as the continuing struggle, not always successful, of a fractious people to overcome prejudice and division and build a stronger, fairer nation. We overcame demagogues on race to carry out a civil rights agenda that responded, imperfectly, to the legacy of slavery and Jim Crow. We have overcome apostles of isolationism to defend democracy and freedom. Repeatedly we overcame ethnic mistrust to welcome immigrants into the fold and see them become the backbone of society.

Today many may be fearful of terrorists, immigration or losing ground economically. But most of us realize that demonizing others won’t solve problems. Mr. Trump is running the campaign of a bigot, a bully and a buffoon. It seems to us it’s worth saying so, if only to make clear that we also know we can do better.