Fellow Atheists, if you were elected President of the US, what would you say in place of "so help me God" ?

Nothing. It’s not a required part of the oath anyway.

“So help me, Goddamn it.”

I would say “help me fellow countrymen”
If I were American that is

I will be long dead before any Atheist is elected president of this country, unfortunately.

As @Harp said, why would anyone say it?

So say we all

or maybe,

By the power of Grayskull

I think it would be interesting if we had Buddhists in politics.

So help me Flying Spaghetti Monster. If you’re gonna remove God there might as well be spaghetti.

So help me, Milo.

If a dyslexic was in office, we might hear “so help me dog”.

Now that I think about it, Milo would be president and say “So help me, Gail.” He says that daily anyway.

“So help me, all of you”

So help me God, you folks elected me president?

So help me Toll House

“Thank you.”

“so help me please”. i believe in manners far more than i believe in christianity.

So help me science and rationality. (I’m not an atheist but I wanted to answer anyway.)

Have you seen that South Park where Cartman freezes himself and goes to the future where everyone is Atheist and says “Science dammit” and stuff. Its funny.

I’m not an atheist either but how about “So, let’s get started”