4. I like the term "religiously unaffiliated". You called it "atheist"
I am both, but of the two, "unaffiliated" is a more accurate description because "atheism" implies I believe something. Instead the supernatural is irrelevant to me. I don't believe it exists so I don't give it a minute's thought most days.
Another way to define this demographic is people who highly value knowledge, education, facts, and reason, and that is not incompatible with a person having some religious beliefs as well. One might have thought that should be pretty near 100% of the population , but we know that isn't the case.
I think we will do better if we focus on people who highly value knowledge, education, facts, and reason. We can be sure we'll get no competition from the GOP on that one. It isn't necessary to have a litmus test as to whether a person believes in the supernatural as long as they have the view that here on Earth, the important things are knowledge, education, facts, and reason. No point in arguing about what may or may not exist in the supernatural. We can't prove anything and it doesn't really change what we have to do here.