The report from Common Cause and The Century Foundation suggests precincts be supplied with more provisional ballots and more electronic voting machines. The report looked at 10 battleground states in the Nov. 4 election.
Florida, Georgia and Virginia have the most potential problems, the report found.
It warned that long lines could discourage voters from casting a ballot in some busy precincts. Georgia's voter registration rolls have surged in recent months.
Common Cause, a nonpartisan group, was the lead plaintiff on the lawsuit challenging Georgia's voter ID law.