Those estimates, from a report published today by the non-partisan Pew Center on the States, portray a largely paper-based system that is outmoded, expensive and error-prone.
"We have a ramshackle registration system in the U.S. It's a mess. It's expensive. There isn't central control over the process," said Lawrence Norden of the Brennan Center for Justice at New York University.
Experts say there's no evidence that the errors lead to fraud on Election Day. "The perception of the possibility of fraud drives hyper-partisan policymaking," said David Becker, director of Pew's election initiatives. But inactive voters do cost money. Inaccurate lists mean wasted money on mailings and extra paper ballots.