The NY Times has an article today where they examine federal election fraud cases over the last five years. Here is the bottom line to their analysis:

Although Republican activists have repeatedly said fraud is so widespread that it has corrupted the political process and, possibly, cost the party election victories, about 120 people have been charged and 86 convicted as of last year.

Most of those charged have been Democrats, voting records show. Many of those charged by the Justice Department appear to have mistakenly filled out registration forms or misunderstood eligibility rules, a review of court records and interviews with prosecutors and defense lawyers show.

In Miami, an assistant United States attorney said many cases there involved what were apparently mistakes by immigrants, not fraud.

At the fraud conference that Mike and I co-hosted in Salt Lake City — we will be publishing peer reviewed papers from the conference in a book early next year — Peter Ordeshook, who studies election fraud in Russian and former Soviet Republics, noted that stealing 1 vote or 10 votes is not fraud by Soviet standards, it is a rounding error. Now stealing a thousand or 10,000 votes, that is fraud.