A subset of voter suppression is voter disenfranchisement, primarily through loss of voting rights due to a felony conviction. Mind you, this is not for felons actually in prison (many states deny prisoners the right to vote while incarcerated), but for those who have done their time and paid the price. It includes even those who have completed parole. And in case you were wondering, voter disenfranchisement was part and parcel of Jim Crow.
While voter disenfranchisement was quite prevalent in the past, only two states now impose permanent disenfranchisement for felons, Kentucky and Virginia. In 2007, Florida restored the voting rights of ex-felons. Alright, I am seeing some improvement. However, those who think this is now a minor problem are first reminded that thousands of ex-felons in Florida were denied the right to vote in the 2000 and 2004 Presidential election.
Spencer Overton, in his book, Stealing Democracy: the New Politics of Voter Suppression (2006), captures the basic rationale for voter disenfranchisement: ex-felons are disproportionately black, brown, poor and undereducated. And roughly 70% vote Democrat. As Overton relates (p.58),
Overton notes how Republican Mitch McConnell, Senate Minority Leader, has opposed voting rights restoration in Kentucky. In 1984 he would have likely lost his Senate race without the help of disenfranchisement. In 1998, Kentucky's other Senator, Jim Bunning barely won reelection, winning by only 6,766 votes. At the time, there were 94,584 disenfranchised ex-felons in Kentucky.
"As frank as I can be, said Alabama Republican Party Chairman Marty Conners in 2003, we're opposed [to restoring voting rights after completion of sentence] because felons don't tend to vote Republican."
Bunning's successor, hard-line ideologue Rand Paul won a very close race in 2010, narrowly defeating his Democratic opponent in one of America's reddest states. I do not have any conclusive figures relating to the possible effect of voter disenfranchisement on the outcome of that race. There is good reason to think it played a decisive role. Some data can be found here. A good starting point on the subject and how the law has changed in recent years can be found at the Prison Policy Initiative.