Donald Trump and Hillary Clinton tell very different stories about who belongs in America and who doesn’t. Trump describes a country under siege from refugees and immigrants, Mexicans, and Muslims. Clinton talks about a nation made stronger by diversity. The narrative each campaign creates matters. It may even influence the way Americans treat their fellow citizens.

A new report from California State University-San Bernardino’s Center for the Study of Hate and Extremism suggests that political rhetoric may play a role in mitigating or fueling hate crimes. The report shows that anti-Muslim hate crimes in the U.S. rose sharply in 2015 to the highest levels since the aftermath of the September 11, 2001 terror attacks. It also suggests that Trump’s anti-Muslim rhetoric could have contributed to this backlash against American Muslims.

“There’s very compelling evidence that political rhetoric may well play a role in directing behavior in the aftermath of a terrorist attack,” Brian Levin, the author of the report said in an interview. “I don’t think we can dismiss contentions that rhetoric is one of the significant variables that can contribute to hate crimes.”

The report from the non-partisan center examined the incidence of hate crimes in the aftermath of two reactions to terrorism from political leaders. First, George W. Bush’s speech following the 9/11 attacks declaring: “Islam is peace” and “the face of terror is not the true faith of Islam,” and the second, Trump calling for a ban on Muslims entering the U.S. after the San Bernardino terror attack. The report found a steep rise in hate crimes following Trump’s remarks and a significant drop in hate crimes after Bush’s speech, relative to the number of hate crimes immediately following the initial terror attacks.

A wide array of factors contribute to the incidence of hate crimes. Ignorance and isolation may play a role; most Americans say they do not personally know any Muslims, although those who do report positive views of Muslims in general. The nature of the threat groups of people are perceived to pose can also be a factor; prejudice catalyzed by a terrorist attack, for example, may be particularly likely to inspire hate crimes. Political rhetoric is only one ingredient in that mix, and the many messages in circulation after an attack can make it harder to determine the impact of any one particular reaction from a political leader. Before Trump’s call for a ban on Muslims entering the country,  President Obama delivered a speech to the nation on the San Bernardino attack stressing tolerance.

Still, the report looked at daily data following terrorist attacks, and found that “a tolerant statement about Muslims by a political leader was accompanied by a sharp decline in hate crime, while a less tolerant announcement was followed by a precipitous increase in both the severity and number of anti-Muslim hate crimes.” It notes that “there have been very few incidents of actual hate crime where Mr. Trump’s name was uttered since his candidacy,” but adds that “the increase of 87.5% in anti-Muslim hate crime in the days directly following his announcement is a troubling development and worthy of concern.”

Aside from calling for a ban on Muslims entering the the United States, Trump has said that “Islam hates us,” and accused American Muslims of protecting terrorists. The research does not demonstrate a direct causal link, nor can it rule out the role of other factors. It’s possible that the documented increase reflects an increase in hate-crime reporting due to heightened awareness of Islamophobia, which has become a topic of discussion during the presidential race. Nevertheless, the research does raise the possibility that Islamophobic political rhetoric may have devastating consequences.

A Georgetown University report released in May similarly found that threats, intimidation and violence against Muslim Americans have surged over the course of the presidential election. Engy Abdelkader, the author of the report, believes that trend is linked to Trump’s political rise. “Trump has seized on people’s fears and anxieties,” Abdelkader said. “I think that has translated in a number of instances not just to hostility, but acts of violence.”

Islamophobia existed in America long before Trump. Muslims have long been particularly vulnerable to backlash driven by negative stereotypes in part because they make up a relatively small slice of the overall U.S. population. But when a major-party nominee endorses and reinforces those stereotypes, researchers warn, American Muslims face serious risk of increased marginalization and outright violence.

When Trump called for a ban on Muslims entering the country, Clinton called the idea “shameful” and “inconsistent with our values as a nation” in a message on her campaign website. She has denounced Islamophobia and made clear that “Islam is not our adversary.” At a town hall in Iowa, Clinton said that “one of the most distressing aspects of this campaign has been the language of Republican candidates, particularly the frontrunner, that insults, demeans, denigrates different people.” She went on to say that “American Muslims deserve better,” adding “we cannot tolerate this and we must stand up and say every person in this country deserves to be treated with respect.”

Still, some Muslim Americans have their frustrations with Clinton. When she condemned Trump’s call for a Muslim ban, she emphasized that it would play into the hands of “radical jihadists.” Advocates caution that political leaders must be careful not to inadvertently amplify potentially stigmatizing associations between Muslim Americans and terrorism. Political leaders should also make clear, they argue, that Islamophobia must be condemned because it denigrates and threatens the lives of American citizens, and not on the grounds that it threatens national security.  

“Muslims cannot just be presented as pawns in this global security game, and that’s how many Muslim Americans feel,” said Laila Abdelaziz, the director of legislative and government affairs for the Council on American-Islamic Relations in Florida. “A lot of the rhetoric coming from the Democratic nominee and Democrats in general is absolutely well-intentioned but it’s important to draw out the problematic and reductionist aspects of it as well.”

Most national-security experts agree with Clinton that Islamophobia can undermine security. “If anti-Muslim rhetoric from Trump and his allies is left unchecked it breeds mistrust and mutual suspicion,” said John Horgan, an expert on terrorist psychology at Georgia State University. “That’s hugely problematic and corrosive to our national security strategy.”

Trust between American Muslims and law enforcement has been strained as a result of surveillance and ethnic profiling, and anti-Muslim rhetoric from political leaders stands to further erode trust. Muslim American communities play a crucial role in alerting law enforcement to terrorist threats, making trust integral to prevention.

The father of Ahmad Khan Rahami, the man charged in connection with bombings in New York and New Jersey, told reporters he contacted the FBI two years ago, though there have been conflicting reports as to whether he contacted the agency directly.

The irony of Trump’s anti-Muslim rhetoric is that the candidate who promises to be tough on terror may be making America not only less safe for American Muslims, but for all Americans.