Levitt also noted that a study of HAVA's voter registration requirements by the Brennan Center found that the databases were quickly created in many states, leaving IT workers little time for training.

'We worry that people will only find out on [Election Day] that they are ineligible to vote,' Levitt said.

Bob Fitrakis, an independent candidate for governor of Ohio, fears that a sizable number of voters have been unknowingly purged from the statewide database there, according to his campaign manager, Paddy Shaffer. The fears were prompted by the use of outdated registration forms in several counties, Shaffer said.

A spokesman for Ohio Secretary of State Kenneth Blackwell did not respond to requests for comment.

In Ohio, said Levitt, eligibility practices are unclear. 'Where systems are less transparent, there's greater reason for concern,' he said.