> One hallmark of the existing protected classes is they are somewhat objective

Another hallmark is they tend to be mostly immutable, or unchangeable voluntarily. Society generally doesn’t accept that it’s reasonable for a person to change their gender, sexual orientation, race, age etc in order to participate in all facets of society.

I don’t think political affiliation meets that standard. Although I imagine that some might consider their political affiliation to be as core their identity as religion.