Donald Trump speaks during a campaign stop Saturday in Birmingham, Ala. (Eric Schultz/Associated Press)

THE GROWING ugliness of Donald Trump’s campaign poses a challenge to us all. We have seen the likes of him before, in the United States and elsewhere: narcissistic bullies who rise to prominence by spreading lies, appealing to fears and stoking hatred. Such people are dangerous.

Like many Americans, our first inclination was to ignore Mr. Trump. The Huffington Post, you may recall, announced that it would feature him in its Entertainment section, not in its coverage of politics. He was a buffoon, a disseminator of ludicrous rumors about President Obama’s birthplace. He lacked the qualifications, experience or knowledge to be president. He was running to promote his brand. We wouldn’t give him the satisfaction.

Our assessment of Mr. Trump was correct, but the tactical response was not. His popular support has hung steady at about 30 percent of Republicans, and his candidacy has tugged the debate toward divisiveness as his bigotry has drawn cheers and many of his rivals have strove to mimic him. Now some detractors opt again to disregard his lies, but for a different reason: Criticism from establishment politicians and tough questions from establishment journalists, it is feared, will only fuel his self-portrayal as the truth-telling outsider.

That may be so, but it cannot justify silence. Just in the past few days, Mr. Trump has repeated the lie that President Obama intends to admit 200,000 Syrian refugees; the correct number is 10,000. He spreads the lie that thousands of American Muslims openly celebrated the 2001 attacks on the World Trade Center; in fact, there were no such celebrations. He tweeted a false statistic that blacks are responsible for 81 percent of murders of white victims; in fact, 82 percent of whites are killed by whites.

These are not random errors. All of them appeal to the basest instincts in supporters; they reinforce fears and prejudices. All of them, Mr. Trump knows by now even if he did not know when he first stated them, are false, but he does not care. The amplification of the lies is accompanied by growing intolerance in his campaign, with Mr. Trump praising supporters for beating a protestor, crudely denigrating anyone who challenges him and penning reporters into designated zones so that they cannot speak with his followers. And all of this matches the brutality of his policies: mass deportation of longtime U.S. residents, torture of foreign detainees, expulsion even of refugees who are here legally .

A few candidates, such as former Florida governor Jeb Bush and Ohio Gov. John Kasich, have spoken out against some of Mr. Trump’s more outlandish comments. Many more, sadly, duck and cover, hoping to avoid Mr. Trump’s insults or to ingratiate themselves with his backers. Sen. Ted Cruz (R-Tex.) is probably the most egregious of these second-rate flatterers, but he’s far from alone. Asked about the mythical cheering crowds, New Jersey Gov. Chris Christie said, “I think if it had happened, I would remember it, but, you know, there could be things I forget, too.”

No, that is not something Mr. Christie would have forgotten, and he ought to say so. Other Republican leaders should speak up too: the speaker of the House and majority leader of the Senate, for example, and former president George W. Bush. The more reticent such leaders are, the more successfully Mr. Trump can brand their party and, to a disturbing extent, the nation with his demagoguery. The only way to beat a bully is to stand up to him.