Are you suggesting that all atheists stand for the "social contract." Is there a voice for all atheists who speaks for you? Or do some atheists believe in different things?
Right now I think atheists are an amorphous, undefined group. I don't think that is necessarily a bad thing, it just makes it tougher to discuss atheism in a political context.
The social contract is the basis of any belief system religious or secular because the question at the heart of every society is how much of personal liberty is the individual willing to surrender in order to have a stable environment in which to live.
The concept of the tabula rasa allows for the individual to formulate a system of beliefs based upon experience and perception. Obviously this can include religious and secular experience. The horror of religion to me is the notion that any one teaching has the more perfect path to salvation and since is the true word of god all non believers are apostates. From this notion of moral superiority flows the crusades, the inquisition, jihad , the second intifadah, the list is endless.
Locke, in his
"Letters Concerning Toleration (1689–92) in the aftermath of the European wars of religion, formulated a classic reasoning for religious tolerance. Three arguments are central: (1) Earthly judges, the state in particular, and human beings generally, cannot dependably evaluate the truth-claims of competing religious standpoints; (2) Even if they could, enforcing a single "true religion" would not have the desired effect, because belief cannot be compelled by violence; (3) Coercing religious uniformity would lead to more social disorder than allowing diversity.[15] "Wikipedia
These are the ideas that heavily influenced the writers of the constitution and any belief system that strays from these principles follows a path of tyranny such is the horror of religion.
While I agree with you that the "social contract" is an important idea, I don't think it unifies all atheists and even if it did, it does not help too much in discussing the politics of atheism.
Some atheists prefer to look at social rules, ethics, morals etc in an evolutionary or Darwinian sense, suggesting that for instance alturism, or obeying societal norms, has a survival value, etc, which is not at all shaped by social contract ideas.
And even if the majority of atheists agreed to the centrality of the social contract, if they varied greatly on the "terms" of said contract you would still have trouble generalizing.