“ Sanders, 376 U.S. 1, 84 S.Ct. 526, without prejudice to the right of the appellants to apply by April 1, 1964, to the District Court for further equitable relief in light of the present circumstances including the imminence of the forthcoming election and 'the operation of the election machinery of Texas' noted by the District Court in its opinion. The stay heretofore granted by Mr. Justice Black is continued in effect pending timely application for the foregoing relief and final disposition thereof by the District Court. ”