Big tent issues combined with opposite extremism.

"Everyone is welcome, so long as they follow these rules" means everyone is welcome, so long as they follow those rules. And that general rule belongs to both sides: So long as you hate the other side and pay lip service to one or two tenants, you're welcome in their faith.

So the left gains people who literally preach genocide, because they 1) Hate the other side, and 2) Hate the right skin colors, religions, and sex. The right gains people who ask to deport natural citizens, because they 1) Hate the other side, and 2) Insist everyone is allowed to say anything while holding any gun, 1A and 2A compliant.

Add onto this big tent idea with the idea that the Right have a long entrenched idea of where the median, the middle ground, of the Overton Window is, and you get Right-wingers looking at the genocidal maniac Left saying "okay, if the Overton window is averaged at zero, and the left and us used to be -2 and +2, and now they're at +10, it makes sense that we need a couple -10's to bring it back to center, we can tolerate them for now."