By the way, worth noting that Montana has same-day registration and North Dakota has no voter registration.
How does a state have no voter registration yet prevent fraud? What stops someone from voting more than once?
The honor system.
Seriously. It's horrible and I have no idea (other than having no especially competitive election) North Dakota gets away with it. Municipalities have the right to maintain voter rolls for local elections, but only one town (a tiny one) does.
Same-day registration has its issues, but the ND system is just totally insane.
