A recent post at Gene Expression looks at the intersection of atheist or agnostic religious views and one's self-identified political stance as a liberal, moderate or conservative from the General Social Survey.
Not surprisingly, self-identified liberals are about three times as likely to be atheist or agnostic as self-identified conservatives, with moderates in between. Indeed, compared to what one might intuitively expect, that religious belief driven skew between liberals and conservatives is smaller than one might expect. About 20% of self-identified atheists and agnostics are self-identified conservatives, while almost half are liberals.
Non-Christians, generally, are considerably more reliable in voting for Democrats rather than Republicans in federal elections based upon exit polls that I've seen over the years. But, atheists and agnostics are only a minority of that category, and some self-identified moderate or conservative atheists and agnostics might still vote for a Democrat over a Republican whose rhetoric, for example, claims that non-believers don't deserve any legal rights. In the same way, Muslim Americans are much more prone to vote Democratic than their views on a variety of political issues would suggest.