Religion

...with the formation of the theocratic National Reform Association.

I suppose you could go back further than that and trace it to the tent revival movement of the early-to-mid 1800s. History rarely works like that, though. You can't typically trace phenomena to one or two antecedent events. I would argue the Christian Right is the product of American Protestantism and populist political thought, and has strengthened over the decades as the cultural rift between rural and urban America has widened.