I highly doubt it. Apparently atheists are trusted about as much as rapists by your average, garden-variety American citizen. Sad as it is, people seem to think that it's actually impossible to be good without god, and I have a feeling that the predjudice won't change soon.
I think it is possible but unlikely. People will say that they have no morality and that a person with no morals can't run the country. Even though that neither statement is true (aethiests do have morals and lets be honest to be a politician you have to leave your morals at the door). But it really isn't about what the truth is, but what people think the truth is. Also people will claim that an aethiest will ban religion, which i think is untrue. Most aethists don't care about what religion people hold just that they don't mess with our religion of non-belief. I think that the aethists candidate would lose a lot of the votes from their party because any die hard religious will not want an aithiest president. I think the attack would be brutal as well. Anyone who runs as an aeithiest would have to be ready to be called all manner of terrible things some that will not even make sense but will all hurt.
Hell no. Did you hear about the fit everyone threw over Obama's potential Islamic beliefs that don't exist? Hell would freeze over before an openly atheist candidate was elected president.
I'm optimistic that this could happen in the future, and hopefully even in my lifetime, but it simply couldn't happen today.
In Tennessee it's still illegal for an atheist to hold public office.