The following is taken from the January 9, 2008, Supreme Court Hearing on voter fraud.

There is no requirement that the State show evidence of past in-person voter impersonation for the State’s interest in preventing such fraud to qualify as important. A State need not wait to suffer a harm; it can adopt prophylactic measures to prevent it from occurring in the first place. That is particularly true in a situation, like voter fraud, where the temptation is obvious and the consequences of undeterred and undetected violations are enormous.

Brief for the United States as amicus curiae, Paul D. Clement, Solicitor General

I suppose. I don’t know if I can say significant. The situation has existed for now a number of years, and the salient fact here is that there’s not a single recorded example of voter impersonation fraud.

Paul M. Smith, Esq.; on behalf of the Petitioners

… if there were that kind of as-applied challenge, one of the virtues of it would be that the remedy at the end of the day would not be to strike the statute down on its face, …

Paul D. Clement, Esq., Solicitor General, Department of Justice, Washington, D.C.; on behalf of the United States, as amicus curiae, supporting the Respondents

Mr. Clement is arguing that the state has the right to be pre-emptive by passing a law to prevent people from committing a crime, even though there are no cases where the crime has been committed. However, citizens do not have the right to be pre-emptive by questioning a law, even though it is obvious that they will be affected if the law stands.

In other words, it is better to prevent a few people from voting because they might vote fraudulently, than to stop a law that will definitely prevent citizens from legally voting. This whole problem could be solved by putting a little purple stamp on voter’s hands. Then everyone would have one, and only one, vote.

http://www.brennancenter.org/dynamic/subpages/download_file_51017.pdf
http://www.supremecourtus.gov/oral_arguments/argument_transcripts/07-21.pdf