Days before accused Buffalo shooter Payton Gendron reportedly posted a diatribe that referenced the “great replacement theory,” an Associated Press-NORC poll suggested that a third of Americans agree with the theory’s premise: that Black and brown immigrants are being brought into America to replace white voters and change the country’s political landscape.

That thinking is especially prevalent among conspiracy-minded Republicans, the poll found.

As such, replacement theory has quickly been picked up as a club to bash Republican candidates who have expressed similar thoughts — and even the GOP in general.

Vice News, which days before the massacre published an article linking GOP candidates (such as Ohio Senate candidate J.D. Vance) to the ideology, went further Monday by saying replacement theory has been “mainstreamed” by the GOP and conservative news outlets like Fox.

Others, however, including conservative commentator Ben Shapiro, have said it’s wrong to conflate a radical conspiracy theory with legitimate and difficult conversations about how immigration and demographic change are affecting America. And the theory didn’t emerge in the cauldrons of conservative media in the U.S., as some liberals have insinuated, but originated in France nearly 50 years ago.

Talk of replacement theory, however, has been been percolating more robustly since last year. In December, the National Immigration Forum issued an explainer that said Fox personality Tucker Carlson had helped spread the idea after thousands of Haitians tried to enter the U.S. through Texas in September 2021.

“While most of the arriving migrants were either turned back into Mexico or deported to destitute conditions in Haiti, some Haitian families were allowed to stay in the U.S. and pursue asylum claims in immigration court,” the National Immigration Forum said on its website, noting that Carlson cited replacement theory on “Tucker Carlson Tonight,” Sept. 22, 2021.

In the segment, Carlson said that President Joe Biden’s border policy aims to “change the racial mix of the country. ... In political terms this policy is called the ‘great replacement,” the replacement of legacy Americans with more obedient people from faraway countries.”

Among cable news viewers, Carlson has tremendous reach. In October 2021, “Tucker Carlson Tonight” was the top show in his time slot among both Republicans and Democrats ages 25-54.

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While there is no one official and singular replacement theory — the school of thought has many variations — it is often distinguished by three components, the National Immigration Forum said: the use of rhetoric that implies America is being invaded and the invasion must be stopped; the idea that pro-immigration policies diminish the power and influence of white Americans; and in the most extreme and antisemitic iterations, that “Jewish elites” are behind the plot.

The advocacy group said that replacement theory emerged from a 1973 French novel, “The Camp of the Saints,” by Jean Raspail, which depicts the collapse of Western civilization because of a massive influx of South Asian immigrants.

NPR reported in 2019 that Stephen Miller, an adviser to former President Donald Trump, had suggested pointing out the novel in conjunction with immigration conversations in the U.S. And the Anti-Defamation League says that the slogan “You Will Not Replace Us” — with the acronym YWNRU — has become a symbol of white supremacists and grew out of “The Camp of the Saints.”

The carnage in Buffalo, where 10 people died Saturday, will forever be linked to replacement theory, given reports that the alleged shooter, who has pled not guilty, mentioned it in a screed.

But the National Immigration Forum noted that replacement theory is not, as some are saying, a creed embraced by all Republicans. Former President George W. Bush, the group said, made one of the strongest statements against white supremacy after Charlottesville, when he said in a 2017 speech:

“Our identity as a nation, unlike other nations, is not determined by geography or ethnicity, by soil or blood. ... This means that people of every race, religion and ethnicity can be fully and equally American. It means that bigotry or white supremacy in any form is blasphemy against the American creed.”

And the AP/NORC Center for Public Affairs research noted that the difference between “high conspiratorial thinkers” and Republicans generally in what Americans believe about replacement theory.

“Despite partisan concerns over immigration, high conspiratorial thinkers are more likely than Republicans generally to believe in replacement theory (42% vs 26%) and express concern the election system discriminates against white Americans (38% vs 25%),” the report said.