> Many may feel that political affiliation should be considered a protected class, however, if it becomes one, would that mean that Nazism is now protected?As tptacek notes, in many ways Nazism is protected through other means. But to directly answer your question, I think the answer is a firm maybe. Because it depends on whether Nazism is recognized as a political affiliation. Which means that that question, too, needs to be settled. (Maybe it is. I don't know.)
That said, I don't think political affiliation should be a protected class (it's something that isn't intrinsic in the way that race or gender identity is and, as tptacek mentioned, can be "taken off" at will) and I don't think it ever practically could be one so I don't think about it that often. Similarly, I also don't really think religious affiliation should be, except that in a practical sense it serves as a shield to protect minority groups as it currently stands so I don't mind the kluge.
> Or does the protection become void as soon as you use it negatively towards another protected class?
I am unclear on the interrelations that you posit, though I just asked a con-law friend for his take and (according to him) it depends on whether or not it is a place of "public accommodation"--retail stores, service businesses, educational institutions. In such a case, yes, it is not lawful to refuse to serve that customer.