TENS of thousands of voters have been purged from electoral rolls or blocked from registering in at least six marginal US states, a report in the New York Times revealed on Wednesday.
The report said that the exclusion of voters appears to violate federal law.
State governments have been trying to conform to the Help America Vote Act of 2002 by removing the names of voters who should no longer be listed.
For every voter added to the rolls in the past two months in some states, election officials have removed two.
The newspaper said that it had identified apparent problems in Colorado, Indiana, Ohio, Michigan, Nevada and North Carolina.
It said that some states were improperly using social security data to verify new voter registration applications and others may have broken rules that govern removing voters from the rolls within 90 days of a federal election.
The Democratic party has been more proactive 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 2000 US presidential election was marked by controversy over the improper removal of voters from the rolls in Florida, which favoured the election of Republican George W Bush.