Suggested reading: _The Truth About Voter Fraud_ by Justin Levitt (2007, PDF, 303 KB) at the Brennan Center for Justice, New York University School of Law. He cites numerous cases of alleged voter fraud that were too insignificant to make a difference in the elections or could not have been prevented by screening voters based on their photo ID (e.g. vote-buying, which rigs the election with the help of valid, eligible voters).