According to a recent op-ed by Brennan Center attorneys Michael Waldman and Justin Levitt, they are about as common:
Before and after every close election, politicians and pundits proclaim: The dead are voting, foreigners are voting, people are voting twice. On closer examination, though, most such allegations don't pan out. Consider a list of supposedly dead voters in Upstate New York that was much touted last October. Where reporters looked into names on the list, it turned out that the voters were, to quote Monty Python, "not dead yet."
Or consider Washington state, where McKay closely watched the photo-finish gubernatorial election of 2004. A challenge to ostensibly noncitizen voters was lodged in April 2005 on the questionable basis of "foreign-sounding names." After an election there last year in which more than 2 million votes were cast, following much controversy, only one ballot ended up under suspicion for double-voting. That makes sense. A person casting two votes risks jail time and a fine for minimal gain. Proven voter fraud, statistically, happens about as often as death by lightning strike.
As a twist, however, they argue that, while individuals rarely engage in voter fraud, a different kind of fraud may be occuring:
Experts have concluded that the most significant threat of fraud comes from electronic voting systems, now used by 80 percent of voters. Legislation introduced by Reps. Rush D. Holt (D-N.J.) and Thomas M. Davis III (R-Va.) would require a voter-verified record along with random audits to double-check against tampering. It would also bar wireless components from machines that could allow a hacker using a PDA to stage an attack. Lawmakers should also immediately stop pushing ID measures that would turn away legitimate voters.
Recent testimony by FBI Director Robert Mueller bolsters Waldman and Levitt's claims:
Schumer: Since 2001, have there been any FBI investigations related to election fraud which you believe should have resulted in an indictment but did not?
Mueller: Not to my knowledge . . . and nothing has come to my level.
Schumer: Has any special agent in charge ever brought such a case to your attention?
Mueller: No, sir.