While neoliberals saw their moves to the centre as inherently conflict-reducing, conservatives saw those moves as deeply threatening. After all, if Clinton or Obama were willing to work with them, there had to be something rotten going on. However, there was no obvious reality-based way to articulate this, since Clinton and Obama actually were quite close to conservative thinking in major ways.
Instead, Clinton and Obama's rightward moves gave rise to two types of conservative reaction. First was an intense explosion of conspiracism, which allowed for the interpolation of vast imaginary political space into a highly compressed political spectrum where no such actual space exists. The militia movement under Clinton and the Tea Party under Obama both traffic heavily in this sort of imaginary, often deeply paranoid politics. (Birth certificates, anyone?)(I would add the persistent gun control conspiracy fantasies to that last item) This is a compelling explanation for the phenomenon. I would like to see more evidence for it, but I'm glad to see at least an idea that attempts to explain the bizarre spectacle of the vast bulk of US conservativism running around claiming their own ideas from just a few years ago are tantamount to Soviet gulags and Nazi health experiments. Rosenberg doesn't mention it, but I think this is pretty compatible with what I remember of Robert Altemeyer's research on Authoritarians - I don't think authoritarians react well to logically similar ideas coming from sources not blessed by their existing authorities. Reacting by moving yet further away from whatever hated group outsider is now endorsing ideas the in-group recently supported is pretty plausible.
It's also productive in that it hints that if Democrats move left, it's possible Republicans will return to something closer to world norms for political conservative movements among industrialized nations. At least, they might not move yet further right than they already are. No reason to assume that this would work both ways, but at least if it's true, Democrats should certainly stop pushing the right, right.