Until his abrupt ouster on Monday, Tucker Carlson used his prime-time Fox News show — the most-watched hour on cable news — to inject a dark strain of conspiracy-mongering into Republican politics.

He's railed against immigration, claiming "it makes our own country poorer, and dirtier, and more divided."

He's called white supremacy a "hoax" and asserted hate speech is "a made-up category designed to gut the First Amendment and shut you up."

As Fox News' "tentpole," drawing around 3 million viewers a night, Carlson's show "has been both a source of that kind of nationalist, populist conservatism that Donald Trump embodied, but it's also been a clearinghouse for conspiracies," said Nicole Hemmer, a history professor at Vanderbilt University who studies conservative media.

Many of the false narratives Carlson promoted were part of the "great replacement" conspiracy theory, the racist fiction that nonwhite people are being brought into the U.S. to replace white voters.

The theory was once considered the fringe territory of white nationalists. But "thanks to Tucker Carlson, this kind of dreck that you would normally only see on far-right forums and online spaces had a prime-time audience on cable news every night," said Melissa Ryan of CARD Strategies, which tracks disinformation and extremism online.

Carlson's show gave many Fox News viewers what they wanted, she said, including false claims about the 2020 election, COVID vaccines and the Jan. 6 Capitol insurrection, as well as smears against gay and transgender people and Russian propaganda about fictitious Ukrainian biolabs.

Carlson and the "4chan to Fox to Trump pipeline"

"Tucker is a chameleon," Ryan said. "He's very good at reading the room and figuring out where the right-wing base is at and adapting to give them as much red meat as they want."

During Trump's presidency, a "4chan to Fox to Trump pipeline" emerged, Ryan said. In one notorious example, a conspiracy theory was circulating on the anonymous message board falsely claiming South Africa was engaging in a genocide against white farmers.

"Tucker Carlson talked about it extensively on the air ... and eventually Trump tweets about it and says that the United States is going to do something about it," she said. "It's sort of insane to think about this content from these forums reaching the president of the United States, and him saying, 'Oh, we're going to act,' we're going to do something about what is a debunked, not true conspiracy theory."

Carlson also gave a platform to controversial figures who shared his conspiratorial worldview — elevating their profiles as well.

"If you had been listening to, say, Alex Jones on Infowars, you would have gotten this material, say, three months before Tucker Carlson got to it," Hemmer said. "But it's showing up on Fox News, which was treated by other news organizations as a legitimate journalistic organization that has millions of more viewers and has viewers who haven't already been radicalized into these conspiracies. That makes Carlson so much more powerful and influential in the broader conservative movement."

Delivering for an audience primed for conspiracy theories

While his most inflammatory screeds sent some big-name advertisers fleeing, Carlson delivered ratings — the primary currency at Fox News.

"Fox News is also very sensitive to what their audience wants and what their audience is saying," Hemmer said. "As that audience has gotten more extreme, as conservative voters and activists have moved even further to the right or have embraced conspiratorial thinking, they've embraced media that give them that," Hemmer said.

Right-wing upstarts like Newsmax and Rumble have expanded the universe of conservative media. But unlike its newer rivals, Fox News still has the reach of a mainstream news outlet.

So when it gives time to extremist conspiracy theories like the great replacement, that reverberates beyond its airwaves.

"Tucker took something that really was relegated to the fringes — it's a white nationalist conspiracy theory — and he made it not just a part of his show, but then a broader part of Fox News's culture, and then, by extension, Republican politics," said Angelo Carusone, president of liberal watchdog Media Matters for America. "It really became acceptable to embrace that idea."

Carlson's final show ended with a promotion for his latest streaming special, called, "Let Them Eat Bugs." In it, he claims that global elites — another staple of Carlson's conspiracies, alongside racial grievance — are trying to force people to replace meat with insects.

"It's part of a larger agenda," Carlson warned.