Republicans like to claim, and have claimed for years, that fraud is the reason they lose or have lost, some close elections.
When challenged for proof, they have none. When their allegations are investigated, such as in the Coleman - Franken race, they evaporate like smoke.
Even the supposedly most credible case about which they bally-hoo and about which you hear, time and time again is the 1960 Presidential election, specifically supposed over-votes in the Chicago area. Please refer to the attached story from Slate.com:
http://www.slate.com/articles/news_and_politics/history_lesson/2000/10/was_nixon_robbed.html
This story debunks that incredible (totally non-credible) claim. The Illinois election was deeply/broadly investigated and while a tiny number of votes (953) were reversed. That's ENTIRELY within the realm of any re-count. There were NO incidents of fraud which were proved. The allegations were vapor, and were litigated and dismissed through.
On a personal level, my father, a man of extra-ordinary integrity, was involved in Chicago politics in the 1960's. He has said the reason for the large majorities of Kennedy (and others) in the 60's was the effectiveness of the Daley "machine." If you had a problem, you called your precinct captain and it got fixed. Consequently people were very loyal to Dems. My father would never, nor would I, nor anyone I know of, EVER have abetted fraud or put up with it, or left it unreported. That's a profoundly offensive allegation. The evidence doesn't support it, and there's no proof you have that it has occurred since (or then). It's the realm of asking someone to prove something didn't happen when it should be you have to prove something DID happen. When asked to prove THAT, the GOP failed utterly. They litigated and they had no proof.
