AS CONSERVATIVES UNIQUELY UNDERSTAND...:

At campaign rallies before last year's midterm elections, President Trump repeatedly warned that America was under attack by immigrants heading for the border. "You look at what is marching up, that is an invasion!" he declared at one rally. "That is an invasion!"

Nine months later, a 21-year-old white man is accused of opening fire in a Walmart in El Paso, killing 20 people and injuring dozens more after writing a manifesto railing against immigration and announcing that "this attack is a response to the Hispanic invasion of Texas."

The suspect wrote that his views "predate Trump," as if anticipating the political debate that would follow the blood bath. But if Mr. Trump did not originally inspire the gunman, he has brought into the mainstream polarizing ideas and people once consigned to the fringes of American society.

While other leaders have expressed concern about border security and the costs of illegal immigration, Mr. Trump has filled his public speeches and Twitter feed with sometimes false, fear-stoking language even as he welcomed to the White House a corps of hard-liners, demonizers and conspiracy theorists shunned by past presidents of both parties. Because of this, Mr. Trump is ill equipped to provide the kind of unifying, healing force that other presidents projected in times of national tragedy. [...]

"The people who carry out these attacks are already violent and hateful people," said Nathan P. Kalmoe, an assistant professor at Louisiana State University who has studied hate speech. "But top political leaders and partisan media figures encourage extremism when they endorse white supremacist ideas and play with violent language. Having the most powerful person on Earth echo their hateful views may even give extremists a sense of impunity."

This has come up repeatedly during Mr. Trump's presidency, whether it be the white supremacists who marched in Charlottesville, Va., or the bomber who sent explosives to Mr. Trump's political adversaries and prominent news media figures or the gunman who stormed a Pittsburgh synagogue after ranting online about "invaders" to the United States.

David Livingstone Smith, a philosophy professor at the University of New England and the author of a book on dehumanization of whole categories of people, said Mr. Trump had emboldened Americans whose views were seen as unacceptable in everyday society not long ago.

"This has always been part of American life," he said. "But Trump has given people permission to say what they think. And that's crack cocaine. That's powerful. When someone allows you to be authentic, that's a very, very potent thing. People have come out of the shadows."

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