A divided U.S. Supreme Court on Tuesday dealt a crippling blow to the Voting Rights Act of 1965 by striking down the formula devised by Congress to determine which states are covered by the act.

“In 1965, the States could be divided into two groups: those with a recent history of voting tests and low voter registration and turnout, and those without those characteristics,” Chief Justice John Roberts Jr. wrote for the majority. “Congress based its coverage formula on that distinction. Today the Nation is no longer divided along those lines, yet the Voting Rights Act continues to treat it as if it were.”