Tens of thousands of eligible voters have been removed from rolls or blocked from registering in at least six swing states, and the voters' exclusion appears to violate federal law, according to a published report.
The New York Times based its findings on reviews of state records and Social Security data.
The Times said voters appear to have been purged by mistake and not because of any intentional violations by election officials or coordinated efforts by any party.
States have been trying to follow the Help America Vote Act of 2002 by removing the names of voters who should no longer be listed. But for every voter added to the rolls in the past two months in some states, election officials have removed two, a review of the records shows.
The newspaper said it identified apparent problems in Colorado, Indiana, Ohio, Michigan, Nevada and North Carolina. It says some states are improperly using Social Security data to verify new voters' registration applications, and others may have broken rules that govern removing voters from the rolls within 90 days of a federal election.
Democrats have been more aggressive at registering new voters this year, according to state election officials, so any closer screening of new applications may affect their party's supporters disproportionately, the Times said.
The result is that on Election Day, voters who have been removed from the rolls could show up and be challenged by political party officials or election workers.
The six states seem to have violated federal law in two ways. Some are removing voters from the rolls within 90 days of a federal election, which is not allowed except when voters die, notify the authorities that they have moved out of state, or have been declared unfit to vote.
And some of the states are improperly using Social Security data to verify registration applications for new voters, the newspaper reported.
"Just as voting machines were the major issue that came out of the 2000 presidential election and provisional ballots were the big issue from 2004, voter registration and these statewide lists will be the top concern this year," said Daniel P. Tokaji, a law professor at Ohio State University.