So here in the Midwest USA, I can go to election commission and legally obtain the voter registration file.
It, by law, is public. Anybody who asks for it must be provided the file. Naturally the law is pre-internet and ignorant of abuses you can do.
But I'm not sure how this leak compares. Is it party affiliations and loads of PII to the point of impersonation?
Yeah, it almost sounds like the leaked information was... effectively the white pages for the area. ie name, address, and phone number
In the UK it's semi-public. The full register is available to politicians and credit reference agencies. You can inspect the register by attending in person at your local council office. You don't get a copy.
There's also the edited register, which anybody can buy a copy of. You only appear in this list if you opt-in when you register. I don't know why anybody does.