The examination of the nation's voter rolls commissioned by the Washington-based center found that 51 million eligible citizens were not registered to vote, more than 24 percent of the eligible population.
Another 2 million deceased people were listed as active voters.
The study also found about 12 million records listed incorrect addresses, meaning either the voters moved or the information contained errors, making it unlikely any mailings can reach them.
"Voter registration is the gateway to participating in our democracy, but these antiquated, paper-based systems are plagued with errors and inefficiencies," David Becker, director of Election Initiatives at the Pew Center on the States, said. "These problems waste taxpayer dollars, undermine voter confidence and fuel partisan disputes over the integrity of our elections."
Becker said election officials from several states have been working with Pew for the past two years on plans to upgrade voter registration systems using advanced technology to get greater accuracy of the rolls, increased savings and improved processes.