Just a broad question, how exactly would one prove citizenship at a polling place? Drivers licenses, voter registration cards are easily forged. So are birth certificates as we've all learned. Utility bill in anybody's name is easy but doesn't mean they live there. Citizenship should be required when the person registers to vote, not at the poll.
Just sayin', if my polling place required proof that I'm a citizen, I have no idea how I would prove that.