Donald Trump speaks at the Family Leadership Summit in Ames, Iowa, on July 18. (Nati Harnik/Associated Press)

DONALD TRUMP’S rapidly expanding catalogue of bombast is already a weighty tome, and it’s a fool’s errand to take each of his utterances seriously. Still, his loathsome comment on Wednesday, in which he excused violence against a Hispanic man in Boston as “passionate” acts of “people who are following me,” taps into a dark vein in American history and merits special attention.

In the Boston incident, two brothers were charged with using a metal pole to assault a 58-year-old Hispanic man. The man, who was homeless, was left with a broken nose and other injuries to his face, arms and chest. “Donald Trump was right, all these illegals need to be deported,” one of the brothers, Scott Leader, told police, the Boston Globe reported.

When Mr. Trump was told of the incident, in which the brothers also are alleged to have urinated on the man before beating him, he said the following: “It would be a shame. . . . I will say that people who are following me are very passionate. They love this country and they want this country to be great again. They are passionate.”

Officials in Massachusetts had a different reaction when the Globe asked them about the incident. “A disgrace,” said Police Commissioner William B. Evans. “Sickening,” said Daniel F. Conley, Boston’s top prosecutor.

Mr. Trump, under a barrage of criticism, took more than a day to retreat from his original statement, finally tweeting that he “would never condone violence” and “we must treat each other with respect.” This from the man who slandered undocumented immigrants as “rapists” and suggested Fox News journalist Megyn Kelly had asked him tough questions because she was menstruating.

Republican presidential candidate Donald Trump released a detailed immigration plan, and its "great, great wall" is just the beginning. (Gillian Brockell/The Washington Post)

The truth is that Mr. Trump deliberately whips up popular rage for political advantage. By spewing hatred on the stump, Mr. Trump encourages it in the bleachers and on the streets, then sanctions it when it occurs. Remember: He also minimized the death threats Ms. Kelly received after his dustup with her, telling the Hollywood Reporter, “I’m sure they don’t mean that,” then pivoting to stress his “respect for the people that like me.”

Mr. Trump is not the first politician to inspire and then explain away crimes of hatred; he’s just the most recent one. Recall the Southern politicians of the past century who were apologists for lynchings. Rep. Charles E. Bennett of Florida, who said he condemned such violence, nonetheless explained that lynchings occurred because Southerners were aggrieved at the meddling of Congress. Others, more coarsely, cited the rape of white women by black men as having naturally incited a lynch mob.

Mr. Trump’s immigrant-bashing rhetoric is intended to galvanize political anger and win Republican primaries, not incite a lynch mob. The trouble is that his contempt-filled hyperbole appeals not to rational discourse but to passions — passions that can and do get out of hand.