We need to significantly reduce the power of the church in this country before that can happen, but I’d love to see the day

I think the question should be, when will America be ready for an OUTSPOKEN atheist President.
I would wager that there has already been a few, but they couldn't admit it because they simply wouldn't get elected.

In terms of authority and leadership, theists will ban together against anyone who believes in
"no god" over "a god". It's a big deal to consider that the office of POTUS is the highest position that exists in the strongest country, or that the office of POTUS still answers to a god.

So my thought would be when ~50% of the US populace is openly atheist would an atheist candidate have a real chance.

I would argue that most Presidents have been atheist, but could not be openly so. As far as when the U.S. will be ready for an openly atheist one, I think it is going to take another few generations. The trend overall is declining religiosity in our population, so I'm thinking it is at least partly a generational thing.

That is a great question, I honestly hope soon. A President that does not even mention the word god would be a nice change.

Some considered Trump the first athiest president. He certainly hasn't lived by any religious principles. That quieted once he say how far the evangelicals were up his bit and he started quoting bible verses. They can have him!

We had some early Unitarian ones. The closest we got. Jefferson put together his own bibly, removing all the supposed words said directly by Jesus. He did a lot of cut and paste. The original still exists. He also owned a number of other religious texts, including a Qur'an which one of the Congressional newbies used.

Thomas Jefferson was what we would call Agnostic. Until he started growing old, when he became a 'deist'.