Now it’s the GOP’s turn. “I can only hope that the Democrats do tone down the rhetoric,” said New York Republican Congressman Chris Collins after the shooting of House Majority Whip Steve Scalise and five others at an Alexandria, Virginia baseball field. “The rhetoric has been outrageous—the finger-pointing, just the tone and the angst and the anger directed at Donald Trump, his supporters. Really, then, you know, some people react to things like that.”

Democrats once said much the same thing. After a Tuscon man shot Arizona Congresswoman Gabrielle Giffords in 2011, progressives noted that her opponent had asked supporters to “help remove Gabrielle Giffords from office” by attending a fundraiser at a shooting range. They also pointed to a graphic distributed by Sarah Palin, which showed Giffords’ district in the crosshairs of a gun. “When people do that,” Giffords said, “they’ve got to realize that there are consequences.”

Yes, there are. Academic research suggests that violent and dehumanizing political rhetoric can increase support for violence among people already predisposed to aggression. For a journal article he published in 2014, Nathan Kalmoe, who now teaches political communication at Louisiana State, asked people to watch different sets of fictional political ads. One set used violent metaphors: “I will fight hard to get our economy back on track,” “I can’t win this battle without your help.” The other set did not: Instead of “fighting hard to get our economy back on track,” the fictional candidate pledged to “work hard.” Instead of winning this “battle,” she talked about winning this “race.”

It made a difference. Kalmoe found that among “citizens with aggressive personality traits” support for political violence “doubled when exposed to political messages infused with violent metaphors.” What’s shocking about Kalmoe’s findings is that the language in the violent ads is so weak. In America today, calling a political contest a “fight” or a “battle” is what George Orwell called a “dying metaphor”: a metaphor that has been so overused that it no longer conjures a visual image. When a politician says he’s going to “fight” to improve the economy, no one imagines people punching each other. Yet even this extremely mild form of violent speech increases support for violence in some people.

Obviously, showing that a hypothetical ad can make people more willing to endorse violence is far from showing that any particular political statement incited James Hogdkinson to open fire in Alexandria. Still, Kalmoe’s research suggests it may matter that Hogdkinson belonged to a Facebook group called “Terminate the Republican Party.” It certainly suggests America would be better off if politicians used fewer martial metaphors.

Only in a country like the United States, where war is overwhelmingly something that happens in far-off lands, can leaders so easily deploy phrases like the “war on poverty” and the “war on Christmas.” Imagine the absurdity of a Syrian or Iraqi politician using such a phrase. At the least, violent metaphors desensitize Americans to the horror of actual violence. At worst, they make such violence more likely.

According to Kalmoe, language that isn’t explicitly violent but “dehumanizes the other side or emphasizes the extremity of provocation by others” may make people more prone to violence too. Such language, he told me, can produce a “moral disengagement that allows people to justify to themselves acts that they wouldn’t otherwise think are moral or ethical.” A recent article in Vox, for instance, cited research by Stanford’s Albert Bandura noting that when people in a study heard researchers calling another study participant “an animal,” they became more likely to give that person a “painful shock.” That’s why Donald Trump’s reference last year to refugees as “animals” was so dangerous. Calling Mexican immigrants “rapists” isn’t a great idea either.  

What’s tricky is determining where legitimate moral outrage shades into dehumanization. Calling Donald Trump’s actions “deplorable” is probably fine. Calling him “deplorable” is a bit more problematic. Placing “half” of his supporters in a “basket of deplorables,” as Hillary Clinton infamously did last fall, is worse.

But as Ta-Nehisi Coates argued at the time, her comments were factually defensible. Three-quarters of Trump supporters backed banning Muslims from entering the United States. Forty percent believed that African Americans are “more violent, more criminal, lazier, and ruder than whites.”

At a time when America’s president, with widespread Republican support, dehumanizes some of the most vulnerable people in the country, how do progressives express their moral fury without embracing a dehumanizing language of their own? Before Wednesday, many liberals may not have considered that an urgent question. Now they have no excuse.