An excellent article by Thomas Sowell on electoral fraud.
And there are plenty of ways to commit fraud Sowell hasn't mentioned. In 2012, a number of Philadelphia city wards reported 99% voter turnout, with 100% of the vote being for Obama. Generally, 70% is considered good turnout for an election, and historically, turnout in the inner cities has been less. So how did all these wards get 99%? It strikes me that it would be awfully easy for the people working those wards to simply check off the names of people who didn't show up to vote and simply fill out their ballots for them.
Or how hard would it be to have people whose job it is to transport paper ballots to a central counting spot to conveniently "lose" the votes for a certain candidate.
How hard would it be to rig an electronic voting machine? There were plenty of reports of people who thought they were voting for one candidate seeing that they had "voted" for another when they tried to use these machines.
How closely do the registrars check to make sure that all of the people who have died recently have been purged from the voter rolls?
As Joseph Stalin said, "It's not the people who vote that count. It's the people who count the votes."