President Trump ignited a racially charged firestorm Wednesday when he retweeted videos from a fringe nationalist party in Britain that purported to show Muslims attacking innocent people, earning condemnation from the British prime minister and political and religious leaders around the globe.

No other modern U.S. president has deployed such inflammatory rhetoric, but Trump has long promoted and disseminated divisive imagery from the far right precincts of the Internet.

The three videos he shared were unverified, but Press Secretary Sarah Sanders said that it doesn't matter: "The threat is real." At least one of the videos, advertised as "Muslim migrant beats up a Dutch boy on crutches," was debunked by the Netherlands government.

Historians and scholars of rhetoric say that Trump's actions are dangerous because they promote extremism and blur the line between fact and propaganda.

"At some point we have to stop saying, 'Look at what he's done,' and say, 'This is what he does,'" Princeton University presidential historian Julian Zelizer said Wednesday. "These are the kind of tweets he enjoys putting out there. And they're dangerous. He brings this stuff into the center of power."

This is, after all, the man who launched his political career by suggesting that Barack Obama was not a U.S. citizen, opened his campaign by calling Mexican immigrants rapists, retweeted someone with the Twitter name "WHITE GENOCIDE," and said that "both sides" were to blame for the violence that left a counter-protester dead at a white nationalist rally this summer.

Indeed, Britain First, the media-hungry U.K. group whose tweets made it onto the President's Twitter account this morning, has been touting the retweets all day. The Buzzfeed UK reporter Jim Waterson described them as a stunt-loving anti-Muslim far-right social media troll group increasingly ignored even in the UK."

"Trump retweeting them is roughly akin to Theresa May retweeting videos posted by David Duke," said BBC reporter Anthony Zurcher.

The Council on American-Islamic Relations called the videos an incitement to violence. "These are the types of materials we see on anti-Muslim hate sites on the internet — we don't expect to see them on the Twitter feed of the President of the United States," said CAIR spokesman Ibrahim Hooper. He said the organization has been concerned all year about growing anti-Muslim sentiment in the U.S., but that the tweets were a "quantum leap."

Imam Salaam Muhsin, of North Philly's Masjidullah mosque, said that such rhetoric makes him feel like the country is leeching a poison.

"[Trump] represents a mentality that has to be exposed — it's racist, it's disrespectful to women. That mentality is not a dominant mentality, not in America and not in the world," he said. "And now that mentality is on public display, and it has to die a public death."

It's hard to find historical precedent for the way Trump talks — and the audience he reaches, said Kathleen Hall Jamieson, a presidential historian at the University of Pennsylvania and a Shorenstein Fellow at Harvard's Kennedy School of Government.

That's not to say that a U.S. president has never expressed xenophobic or racist views — Jacob Bender, the director of the Philly chapter of the Council of American-Islamic Relations (CAIR), cited Woodrow Wilson's committed segregationism and Andrew Jackson's boasts about the Indian Removal Act. Zelizer said George Wallace, the former governor of Alabama and 1968 presidential candidate who deployed caustic rhetoric on race throughout his run for the White House, is the closest contemporary analogue to the way Trump talks about race, religion, and immigration.

But no one had the reach Trump does — past presidents couldn't tweet their views to millions, instantly — and they have generally carefully vetted the public statements they do make, especially on international issues, Jamieson said.

Jamieson likened Trump's retweets to wartime propaganda with its dehumanizing, stereotyping effect on groups identified as the enemy. But even then, she said, "We don't have instances in World War I and World War II where our allies responded to US propaganda by saying, 'What are you doing?'" (May, the British prime minister, condemned the tweets on Wednesday.)

There are many dangers inherent in spreading tweets like the ones Trump publicized on Wednesday, she said. Pushing unvetted information weakens the country's stance abroad. Engaging in culture wars on Twitter distracts the news cycle from legislative behemoths like the tax bill that deserve scrutiny. And it normalizes rhetoric that the public once largely condemned.

"I'm very concerned about what happens when things feel as if they are normal, when they shouldn't — when they seem as if they are acceptable because you keep seeing them in public space, even when people say they shouldn't be there," Jamieson said. "Creating fear of others is not a useful strategy in a democracy to rally one's base. It destroys our sense of the community that is the country."