It's very depressing, but getting elected as a self-confessed atheist or agnostic probably will remain difficult for many years time. Apparently no members of Congress are currently openly atheist, and it was only in 2014 that the first ever openly atheist candidate even ran for Congress.
Several opinion polls have been published which suggest gays, Muslims, pot smokers and adulterers all stand a better chance of being elected in America than atheists!!! (I don't have anything against gays or Muslims - or perhaps even pot smokers - but even so, that seems a bizarre statistic).
Why is America so mistrustful of atheism and agnosticism, despite the fact that a very significant minority (up to 20%) of Americans do not believe in God? Is it because they think atheists lack morality? Despite knowledge of the countless millions who have died or been persecuted with intolerance by those of faith?
The prejudice against people who choose to live by research- based evidence rather than the words of religious texts written more than a millenium ago, causes raised eyebrows in almost all other enlightened countries. In these countries, science and evolution have become accepted as facts which believers simply have to reconcile with their faith, and atheism is recognised as being not in any way detrimental to moral systems of government.
There will eventually be an atheist president, because sense will eventually prevail, but perhaps not in my lifetime.