...it's the emergence of a coherent conservative ideology combined with a slight leftward shift among Democrats on social issues (identity politics?). The dominance of New Deal liberalism in the immediate post-war years in concert with the lack of any consistent worldview among Republicans created the appearance of moderation, but not its substance. Contemporary liberals like to reminisce fondly of the days when the parties were more 'moderate,' but what they really mean is that the miss the days when the US was basically a center-left society drifting towards European-style social democracy. Calling a period without a strong opposition party 'moderate' doesn't make it so.