The KKK is only the most extreme example of anti-Catholicism in American history - and given the popularity of Birth of a Nation, it is clear it enjoyed some support from Protestants. But, even before the KKK, in the 1840's a major American political party (the American Party, better known as the "Know-Nothings") had as their centerpiece an anti-catholic platform.
I do think that the comparison between Harley and the KKK (which was the poster's intent) was beyond the pale. But I've been wrong before.