Many Republican controlled states have passed voter fraud laws, which make it much harder for people (cough, Democratic-leaning voters) to vote.
Electoral Vote has a short, pointed article about voter fraud, pointing out what a fraud it is. Here’s an excerpt (and by excerpt, I mean about half the whole article):
From time to time we see an article in which the author cites the case of someone who impersonated a voter as a test and got a ballot. From one or two instances, the author concludes in-person voter fraud must be rampant. An analogy could be made with tiger teams trying to smuggle weapons onto airplanes. Often the latter works. But there is a huge difference between the two exercises. If one person smuggles one weapon onto one plane, it could result in a plane crash, with hundreds of people being killed. If one fraudulent voter manages to vote illegally, it is not going to change an election result, except maybe for dogcatcher in a town with seven voters.
Consider what it would take to actually change, say, a congressional election. The closest congressional election in 2012 was in IL-13, where Rodney Davis (R) defeated David Gill (D) 137,034 votes to 136,032. To flip that election, Gill would have had to get 1,003 people to vote for him illegally. How would one recruit 1,003 people to cast fraudulent votes? Place an ad on Craigslist or in the local newspaper? Probably not a great idea, since asking people to commit a felony is not something you want to get caught doing. Maybe a radio spot? Attend a meeting of the College Democrats and pass out a flyer? Remember, this has to be done very covertly. Once you explain to an interested party what you want, most of them are going to sense you are asking them to commit a crime, even if they don’t know the exact penalty. Most likely you are going to have to talk to thousands of people to get 1,003 who agree. Obviously, there is a great danger that more than a few of the people who you approach and who say “no” might go to the police.
In short, even flipping the closest congressional seat would be an extremely difficult and risky process, with thousands of people knowing about it, any of whom could expose the scheme. When considering Voter ID laws, and other “anti-fraud” measures, this microscopic chance of fraudulent voting changing an election result has to be weighed against the very real possibility of thousands of actual voters being disenfranchised because they lack voting credentials and are unable to overcome barriers intentionally put in place to make it difficult to get them.