From the November 2012 issue of Harper's, of which I am a proud print subscriber.
"How to Rig an Election: The G.O.P. Aims to Paint the Country Red" by Victoria Collier
An in-depth article on election fraud, past and present
At the time of publication the article was behind a paywall, but it became publicly accessible few days before 2012 US Presidential Election. Links to the article (not counting blogspam) were posted 26 times on Reddit (link for the submission with the longest discussion thread and most upvotes). There was much handwringing over potential and actual voter disenfranchisement in the days leading up to Election Day.
Collier focuses on the potential for election fraud inherent in the widespread use of proprietary black box electronic voting machines. In the days of Tammany Hall and during the senatorial election of Lyndon Johnson, there was always evidence of the crime - ballot boxes never delivered or obviously fraudulent paper ballots; whereas the machines in use across the US since the mid-1990's show no signs of tampering. The vulnerability of these systems is striking given that many are produced by makers of ATMs. Such vulnerability has been demonstrated ad naseum by well-respected authorities such as Bruce Schneier. Not only the voting machines themselves, but the infrastructure through which they transmit the results to central servers is vulnerable to man in the middle attacks.
Statistical irregularities and certain events in the aftermath indicate that electronic voting systems were manipulated in the 2004 US Presidential Election and the defeat of an incumbent Democratic senator in 2002. In each case, the vote tally percentages for the (eventual) losers decreased through the day on election day, but only in key districts and only where electronic systems were used. This phenomenon is so common it has a nickname: red shift, since the votes almost always flow toward the Republican side (although not exclusively; there have been a few rare cases of blue shift).
Notably the flipping of votes is never done in bulk but in small amounts here and there, not enough to guarantee a win but enough to tip the balance in a close race. Add to this the fact that exit polls conducted by leading news organizations now incorporate the latest tallies in their statistical projections, which makes it even more subtle. The wider public often accepts the reported outcome regardless of inconsistencies, especially now that the shifting tally is given weight in the predictions reported by the news media!
Vote rigging is only one tactic of disenfranchisement; others we have seen this election cycle include false advertising, intimidation, changes to registration laws such as requiring certain forms of ID to register and/or vote, and selectively purging voter rolls. This issue of Harper's also has a feature on past and recent instances when the Voting Rights Act of 1965 has been enforced. The law, drawing its power from Congress and the 15th Amendment, struck down Jim Crow laws such as those requiring a citizen to pass a literacy test to be eligible to vote.
The wider public and news media have often dismissed charges of vote-rigging as conspiracy theories. To the point the article answers:
"The public unmasking of [Diebold's proprietary tabulation program] by an average citizen (who was not a
programmer herself) served as a belated wake-up call to the world’s
leading computer-security experts, who finally turned their attention to
America’s most widely used voting systems. Damning reports have since
been issued by researchers from Johns Hopkins, Princeton, Rice, and
Stanford Universities, the Brennan Center for Justice, and the
Government Accountability Office (none of them institutions hospitable
to “tinfoil hat” conspiracy theorists)."