I've been a Republican, a Libertarian, and now a Democrat (as of 2016). As far as I can tell my atheism was not a factor. I was an agnostic theist when I was a Repbulican and when I became a Libertarian; I became an atheist while I was a Libertarian. I feel like my political evolution was due to education more than anything else. I don't believe in spirits or eternal souls, so I'm not worried about embryos or fetuses having souls; but that's about it. I might be pro-choice even if I did think fetuses have souls.
I'm not anti-Christian. I'm anti-stupid.