Heck had there ever been an atheist Democrat senator?
Thomas Gore.
As far as I know, we have no (open) atheists in the chambers right now, despite Krysten Sinema being well known to refuse to identify with a religion. She has denied the label of atheist.
OP is out of the question for a long time, but were a state to elect an atheist Republican today, the states that would be the best candidates would be New England states.
What a coward.
I'm not sure if she's said anything that would cast doubt on this, but based on what I know about Sinema, she could very well not
be an atheist. Atheists aren't the only people who don't identify with a religion.