Odds are, the Supreme Court will strike down a key provision of the Voting Rights Act after hearing a case from Alabama that will be argued next month. If the part of the law called Section 5 does indeed go down, minority voters in Southern states and elsewhere will lose a key bargaining chip. Section 5 has enabled them to beat back some attempts to make it harder for them to vote, and helped insure that the gains they’ve made in representation and redistricting are not rolled back. As another recent fight over South Carolina’s voter ID law shows, Section 5 still serves a vital role in an era in which partisan legislatures may manipulate election laws for political gain

Like many other states with Republican majority legislatures acting over the last few years, South Carolina adopted a tough photo identification law before the 2012 election. The state’s Republican legislature likely acted out of the belief that such laws would marginally depress Democratic turnout and help Republicans at the polls. Controversy over voter ID laws also motivates the Republican base to turn out to vote. (What voter ID laws don’t do is prevent a lot of real voter fraud, though that’s the rationale their supporters cite.)

The U.S. Department of Justice blocked South Carolina’s voter ID requirement under Section 5. The process sounds technical, but it’s important. Nine full states and parts of other states with a history of racial discrimination in voting must get approval from either the Department of Justice or a three-judge court in Washington, D.C. before making any changes in their voting practices and procedures—from changes as small as moving a polling location to as large as enacting a new redistricting plan.

The states and localities covered by Section 5 must prove that any change in voting rules it proposes will not have the effect, and was not enacted with the purpose, of making minority voters worse off. The original point of this law, when it first passed in 1965, was to stop an old cat-and-mouse game in which the federal government sued racist Southern states to stop discriminatory voting practices—and after the feds won, the states would just enact a slightly different discriminatory law in its place. Today, the law prevents these jurisdictions from slipping back into that pattern.