Every thorough study of voter impersonation has found it to be exceedingly rare.
Lorraine Minnite of Rutgers University notes that in 2005, at the peak of a federal crackdown, “federal prosecutors indicted far more people for violations of the nation’s migratory-bird laws than for election fraud.”
A comprehensive national study by the Walter Cronkite School of Journalism at Arizona State University scrutinized thousands of allegations; it found ten examples of in-person voter impersonation over a dozen years.
A draft study for the federal Election Assistance Commission conducted by Democratic and Republican experts concluded, “There is widespread but not unanimous agreement that there is little polling place fraud, or at least much less than claimed, including voter impersonation, ‘dead’ voters, noncitizen voting and felon voters.” Justin Levitt calculated that statistically “it is more likely that an individual will be struck by lightning than that he will impersonate another voter at the polls.”
http://billmoyers.com/story/whats-behind-the-gop-witch-hunt-for-voter-fraud/