Based on his online manifesto, the alleged killer whose shooting spree in an El Paso Walmart left 22 dead seems to have subscribed to a resurgent racist theory, elements of which have been picked up in recent years by and propagated in the conservative mainstream.

The Great Replacement theory, a favorite of white nationalist conspiracists, posits that elite leftists are plotting to repopulate majority white countries with foreigners, almost always of color.

The conspiracy theory appears to have seeped into the ether of the conservative movement. Some figures on the far-right openly subscribe to it, while others closer to the mainstream knowingly or unknowingly echo elements of the theory.

But it’s gained attention over the past year as various mass murderers — including the Pittsburgh synagogue and El Paso shooters — have referenced the idea in manifestos that seek to glamorize the motivations behind the massacres.

“I am simply defending my country from cultural and ethnic replacement brought on by an invasion,” the El Paso shooter wrote.

The conspiracy theory has evolved from a fringe European idea into a concept that’s been customized for American politics, becoming a main talking point of the alt-right, while Fox News and, occasionally, the President have drawn on elements of it without necessarily fully adhering to the notion.

The American version of The Great Replacement theory draws heavily on fears of a supposedly darkening Europe, with American proponents arguing that economic decline and immigration constitute a slow-burning “white genocide.”

According to Anti-Defamation League senior research fellow Marilyn Mayo, the main difference between U.S. and European iterations of the theory boils down to the groups “targeted as enemies in terms of The Great Replacement.” In the U.S., adherents are worried about Latinos in addition to the Muslim immigrants that European right-wingers denounce.

The idea was described in 2011 by French author Renaud Camus, Mayo said, who wrote an essay called “The Great Replacement” which argued that “replacist” elites were attempting to replace the white population of Europe with African and Arab Muslims.

So how did the theory go from being the province of racist French intellectuals to justifying mass murder at an El Paso Walmart?

Far-right groups in the English-language world grabbed onto Camus’s statement of the theory after its publication. The El Paso shooter claimed to have read the book in his manifesto, while translations of the book reportedly became popular among the online alt-right.

In July 2017, white nationalist Youtube star Lauren Southern posted a video called “The Great Replacement” that has since garnered more than 670,000 views.

In the monologue, Southern blends fear-mongering about the supposed Islamification of Europe with specifically American concerns about automation and disappearing jobs to create a narrative of whites being replaced.

“The future of Europe looks pretty halal,” she says, before discussing how technology makes jobs — and people — redundant. “It’s all replaceable and there’s nothing particularly significant about the thing being replaced.”

According to Keegan Hankes, the interim research director of the Southern Poverty Law Center’s Intelligence Project, the theory is a “dressed-up version” of older ideas that white nationalists have peddled about supposed conspiracies to destroy the white race.

“It’s not that the ideas are new, it’s new packaging,” he said.

But the idea has caught fire on the U.S. far-right since Trump’s election.

White nationalists chanted “Jews will not replace us” and “you will not replace us” at multiethnic counterprotestors in Charlottesville in August 2017. They may or may not have known the theory’s history, but elements of the idea appear to have influenced them.

Over the past year, the idea of dark-skinned foreigners replacing America’s white population has continued to seep into mainstream conservatism, in ways often inspired by fear of a supposed catastrophe in Europe.

In August 2018, Rep. Steve King (R-IA) explicitly endorsed the theory, telling a far-right Austrian website, “Great Replacement, yes,” while adding that “These people walking into Europe by ethnic migration, 80 percent are young men.” King went on to blame billionaire George Soros as masterminding the supposed cabal behind the conspiracy.

Fox News has begun to propagate aspects of the racist narrative over the past year, without necessarily adhering to the theory itself. The Fox News version is more narrowly focused on domestic electoral politics, accusing Democrats of pushing for replacement for partisan political advantage.

Laura Ingraham has subscribed to the notion that Democrats are trying to use “chain migrants” to replace Americans. She said in one appearance last year that Democrats “want to replace you, the American voters, with newly amnestied citizens and an ever-increasing number of chain migrants.”

Tucker Carlson has also echoed elements of the theory — “demographic replacement” is his term — using it as a partisan bludgeon to accuse Democrats of trying to seed the country with liberal constituencies.

After the Brett Kavanaugh hearings, for example, Carlson claimed that leftists were advocating for a “genocide” of white men. He has also characterized the difference between U.S. policy towards immigrants and towards its native-born citizens as “it’s like, shut up, you’re dying, we’re gonna replace you.”

In making that argument, Carlson has ventured deep into the fever swamps. He has claimed that Democrats want “demographic replacement” with a “flood of illegals” to create “a flood of voters for them.”

Hankes argued to TPM that demonizing immigrants is “inseparable” from the idea of a supposed “replacement.”

“This idea that a great cultural or ethnic replacement is coming couples very, very well,” he said.

But it’s President Trump that has provided the highest-profile use of rhetoric associated with the idea of The Great Replacement. He has described Muslim and Hispanic immigrants as “invaders” and criminals. His campaign has ran thousands of advertisements about a supposed immigrant “invasion.”

On Tuesday, Fox & Friends host Brian Kilmeade defended Trump’s use of “invasion,” saying “that’s not anti-Hispanic. It’s a fact.”

“What that does for white supremacists is that it makes them feel they have more license to act,” said Mayo, the ADL researcher.

Apart from the El Paso shooter, other mass murderers have cited The Great Replacement theory or elements of it as underpinning their decision to kill.

The Christchurch shooter cited it in his manifesto, while the Pittsburgh Synagogue killer made a series of anti-Semitic posts before the October 2018 massacre that accused Jews of trying to “bring invaders in that kill our people.”

There are no signs that the theory will abate in influence as time goes on. On the Monday after the shooting, the FBI issued a warning that the threat of homegrown violent extremism continues.

“The FBI remains concerned that U.S.-based domestic violent extremists could become inspired by these and previous high-profile attacks to engage in similar acts of violence,” the warning reads.

For Hankes, the Southern Poverty Law Center staffer, the use of this rhetoric in the White House and mainstream conservative media does the job of white supremacists for them.

“The position for a lot of white supremacists is that they don’t have to say these things as loudly if they’re being championed by the President of the United States,” he said.