Question about voter purging

[nxylas]

Once again, I need to ask a question that most of you have probably known the answe to since high school (my excuse, as always, being that I am an immigrant who has only been in this country 5 years and doesn't know all the minutae of election law). My question is this: is there a deadline for the Republicans to purge likely Democratic voters from the voter rolls, or could they do it the day before the election, meaning that even people who double-checked to make sure they were registered could still be turned away on election day?

[Eric J in MN]

1. Legally, they are not allowed to do "systematic" purges 90 days before...

...a federal election.

States interpret "systematic" in different ways.

And some states are just ignoring that law.