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.  […]