Fox News host Tucker Carlson has refused to walk back his claim that immigration makes the U.S. “poorer and dirtier,” which has caused several companies to pull ads from his show.

The “Tucker Carlson Tonight” host on Monday doubled down on his racist rhetoric from last week about the migrant caravan in Mexico as he told viewers: “It’s true.”

“The left says we have a moral obligation to admit the world’s poor. Even if it makes our own country more like Tijuana is now, which is to say poorer and dirtier and more divided,” Carlson said.

“That’s what we said. It’s true, ask Genaro Lopez,” he added. “Thanks to the efforts of the American left, he and his city are living with the consequences. But precisely because it is so obviously true, saying it out loud is a threat.”

We spend a lot of time talking about the threat to free speech. It’s not an academic question. If they can force you to shut up, they will. Here's their latest attempt. (PART 1) pic.twitter.com/qckMRqFZVj — Tucker Carlson (@TuckerCarlson) December 18, 2018

We spend a lot of time talking about the threat to free speech. It’s not an academic question. If they can force you to shut up, they will. Here's their latest attempt. (PART 2) pic.twitter.com/wh3blwCz26 — Tucker Carlson (@TuckerCarlson) December 18, 2018

We spend a lot of time talking about the threat to free speech. It’s not an academic question. If they can force you to shut up, they will. Here's their latest attempt. (PART 3) pic.twitter.com/TXiLJtoXzn — Tucker Carlson (@TuckerCarlson) December 18, 2018

“It is a shame that left wing advocacy groups, under the guise of being supposed ‘media watchdogs’ weaponize social media against companies in an effort to stifle free speech,” a Fox News spokesperson said. “We continue to stand by and work with our advertisers through these unfortunate and unnecessary distractions.”

Carlson also railed against the businesses ― including jobs website Indeed and life insurers Pacific Life ― that in recent days have yanked their sponsorship of his show.

“Those who won’t shut up get silenced,” Carlson said. “The enforcers scream ‘Racist!’ on Twitter until everybody gets intimidated and changes the subject to the Russia investigation or some other distraction. It’s a tactic, a well-worn one. Nobody thinks it’s real. And it won’t work with this show. We’re not intimidated. We plan to try to say what’s true until the last day.”

This article has been updated with comment from Fox News.