In the twentieth century, the Court interpreted the amendment more broadly, striking down grandfather clauses in "Guinn v. United States" (1915). It took a quarter century to finally dismantle the white primary system in the "Texas primary cases" (1927–1953). With the South having become a one-party region after the disfranchisement of blacks, Democratic Party primaries were the only competitive contests in the states. But Southern states reacted rapidly to Supreme Court decisions, often devising new ways to continue to exclude blacks from voter rolls and voting; most blacks in the South did not gain the ability to vote until after passage of the mid-1960s federal civil rights legislation and beginning of federal oversight of voter registration and district boundaries. The Twenty-fourth Amendment (1964) forbade the requirement for poll taxes in federal elections; by this time five of the eleven southern states continued to require such taxes. Together with the US Supreme Court ruling in "Harper v. Virginia State Board of Elections" (1966), which forbade requiring poll taxes in state elections, blacks regained the opportunity to participate in the American political system.