That isn't especially related to my original point, because it seems specific to the current structure of U.S. politics; it's not very applicable to countries that don't have a few large long-lived political parties. For instance, in Israel, many people were born before the establishment of the state, and no party has survived since then.

We need to look at aspects of U.S. political belief to make good U.S.-specific predictions. And in that, you are surely better informed than I am. So I accept your conclusion that in that context, intelligence is causative of rational and of moderate beliefs. But the specific reasons and dynamics that lead to that seem highly contigent.