@salixlucida It's because modern American conservatism is rather specific form of biggotry.
@jautero I think it's both. The Overton Window for the left is moving leftward and the Overton Window for the right is moving right. Basically it is the death of a long-standing American tradition of centrism in which the partisan differences were not as stark as they are now. The resulting middle vacuum could prove to be a viable space for minor parties if they intentionally target the middle (like the Independent Party of Oregon).
@salixlucida Though, is it American society shifting left or American conservatism shifting right?