My own theory of why atheist relationships scare people: personal or political

[Curmudgeoness]

3. “Voters warm to the idea of atheist political leaders.”

Putting that caption on a photo of Bernie is dishonest. From all that I have read, Bernie is not identifying as an atheist. He has stated that he does believe in a god, just not the traditional way that most people see god. That is not an atheist. But I am sure that those opposed to Bernie want to put that label on him.

But back to your OP, I do think that atheists are a "tribe". We have something to bind us together. However, we don't have enough atheists for us to bond with in the political theater. We do put that out of the equation when we are selecting who to support, although I will say that a hard core religionist would not get my vote. We do evaluate on other issues.