FDR had tried to pass legislation that would let him add up to six more justices. This was because the court had struck down some of his New Deal legislation and so he wanted more power. As shown though with the theory of separation of powers, it would have to go through Congress, who refused to allow the clause that let him "pack the courts" and as a result only reform for lower courts was passed and FDR failed in his main goals, but over his twelve years still appointed most of the justices that were on the court by the time his tenure as president ended.
