It’s probably surprising, but Federally and in all states I know, Political Discrimination is not an EEOC matter.
It’s legal.
Some employees could probably try to make a civil case, but it would be tough. Certainly telling someone how to vote is not actionable, because in the voting booth, that employee can do what ever they want.
Firing may be another matter.
Retaliation for exercising a right is always illegal.
Telling them not to vote likely wouldn’t do it, but telling them to commit voter fraud and firing them if they didn’t definitely would in most states I’m aware of. That’s really about it as far as this issue is concerned.