Fifty years ago, on August 6, President Lyndon B. Johnson signed the Voting Rights Act of 1965 into law — and helped millions of black Americans finally register to vote without the impediment of Southern states' Jim Crow laws.

The Voting Rights Act allowed the federal government to dismantle state-level measures that made it very difficult or even impossible for African Americans to vote, including poll taxes, literacy tests, and outright violence against black voters.

The law had a huge impact on many Southern states. For example, black voter registration rates in Mississippi increased from a mere 6.7 percent in 1965 to 59.8 percent in 1967, according to the US Commission for Civil Rights. For a state that's historically around 40 percent black, this represented a massive shift in politics — a change that much of the predominantly white leadership at the time feared but would have to accept due to the Voting Rights Act.

But the landmark legislation didn't immediately dissolve all voting disparities. In 1967, there was still a roughly 32-point gap between black and white voter registration rates in Mississippi.

The gap continued to close over the next several decades in Southern states. In 1988, Census data for Mississippi showed a 6.3-point gap as white voter registration fell and black voter registration increased. In 2012, Mississippi's gap even reversed: 90.2 percent of black voting-age residents registered to vote, compared with 82.4 percent of non-Hispanic white voting-age people.

But a 2013 Supreme Court case could put a stop to the voting demographic trends. In Shelby County v. Holder, the nation's highest court struck down part of the Voting Rights Act, eliminating the federal government's ability to block states' voting restrictions. In response, many states have stepped up the passage of new voting restrictions, such as voter ID laws, that disproportionately impact minority groups.