In the Shelby County, Alabama v. Holder case, the Supreme Court found that Section 4 of the Voting Rights Act of 1965 (VRA) was unconstitutional. Section 4 sets out the formula by which the electoral processes of certain jurisdictions are placed under the purview of the U.S. Department of Justice (DOJ), which is further laid out by Section 5 of the VRA. Essentially, those states, counties, cities, and special voting districts (such as water and conservation districts, etc.) who have had a history of discriminating against people of color had to submit any changes in their electoral processes to the DOJ, and changes would only be approved once the DOJ was satisfied that the change did not impair the democratic participation of communities of color.
In the 2012 election cycle alone, Section 4 and Section 5 of the VRA worked in tandem with one another to block restrictive voter ID programs in Pennsylvania, Wisconsin, South Carolina, and Texas. In the South Carolina case, the courts found that thousands of Black voters would have been disenfranchised by the institution of a restrictive form of voter ID. It makes sense; the people who are the least likely to have a valid photo ID on them are the typically poor, students, or the elderly. Even if photo IDs are offered for free, people who do not have them still have to travel to their nearest DMV to obtain them. This puts a burden on those who cannot afford transportation.
The vote in support of repeal came from the usual conservative five (Thomas, Alito, Kennedy, Scalia, and Chief Justice John Roberts) versus the usual liberal four (Breyer, Ginsburg, Kagan, and Sotomayor) who voted to uphold Section 4.