“Purging voter rolls, however, can. Yet when the State Board of Elections took steps last year to clean up the voter rolls and asked localities to remove names of those no longer eligible from the rolls, the state Democratic Party sought an injunction to stop the process, and Chesterfield’s registrar, Lawrence Haake, declined to participate, citing inaccuracies. Haake might have been justified, and the board might have pushed too hard to execute what should be a painstaking process. But when those particular concerns are set aside, the general point stands: Localities should keep their voting rolls as up to date as reasonably possible.”