The Supreme Court had declared the National Industrial Recovery Act of 1933 (NIRA) and the Agricultural Adjustment Act unconstitutional, and the Wagner Act and the Social Security Act were pending decisions.

FDR believed that that the conservative court was partisan and would strike down nearly all the major New Deal Programs that had been introduced to combat the Great Depression. The purpose of court-packing plan was allegedly to increase the Court’s efficiency but FDR wanted to use the plan to appoint justices who would not block his administration’s New Deal programs. FDR's actions forced the Supreme Court to back down but the controversial plan ultimately failed through lack of support cost FDR significant political capital in the process.

Facts about Court Packing Plan
The following fact sheet contains interesting facts and information on Court Packing Plan.

The innovative New Deal programs aimed at Relief, Recovery and Reform were extremely popular with the majority of Americans who were gripped in the Great Depression.

FDR was furious that the Supreme Court had ruled the Agricultural Adjustment Act and the National Industrial Recovery Act (NIRA) that had been passed in 1933 were unconstitutional and that decisions regarding the validity of the later Social Security Act of 1935 and the Wagner Act were also pending judgment by the Supreme Court.

In the Judiciary Act of 1869 Congress had established that the US Supreme Court would consist of the Chief Justice and eight associate judges. FDR referred to the justices as "nine old men". At this time it was the most elderly court in the nation’s history, with four judges over 70 years old and two judges in their late 60's. The total ages of justices averaging 71 years .

FDR had known that four of the justices, James McReynolds, George Sutherland, Pierce Butler, and Willis Van Devanter, referred to as "the Four Horsemen", would vote to invalidate almost all of the New Deal. In the spring of 1935, a fifth justice, Hoover-appointee Owen Roberts, began casting his swing vote with “the Four Horsemen” to create a conservative majority. Occasionally Chief Justice Charles Evans Hughes was also in agreement with the majority, conservative justices

It seemed likely that the conservative justices of the Supreme Court would strike down nearly all the major New Deal programs. FDR worked with Attorney General Homer Cummings, on a plan to ensure a more favorable response to the New Deal from the Supreme Court.

FDR knew that he was moving into dangerous waters as the majority of Americans believed the Supreme Court sacrosanct. The president never mentioned the Supreme Court during his campaign for reelection.

President Roosevelt was re-elected in a landslide victory and decided to 'flex his muscles' to curb and change the political balance on the Supreme Court.

President Roosevelt proposed the Judicial Procedures Reform Bill of 1937, or the 'court-packing plan  to Congress on February 5, 1937. The central provision of the bill would have granted the President power to appoint an additional Justice to the U.S. Supreme Court, up to a maximum of six, for every member of the court over the age of 70 years and 6 months. If the bill was passed it would therefore allow FDR to quickly appoint as many as 6 new judges.

FDR did not want to be seen to attack the court so he took the approach of capitalizing on the public’s concern about the ages of the justices and claiming that they were over burdened with work.  .

The bill was the subject of one of President Roosevelt's Fireside chats on March 9, 1937.

The plan backfired and was FDR's first serious political mistake. 

The scheme gave the impression that the president was undermining the court's independence and interfering with the Constitution's Separation of Powers. Americans were concerned that the scheme would give the President too much power

When the Judicial Procedures Reform Bill went to Congress the scheme lost support and split the Democratic Party. Southern Democrats expressed concerns that new justices appointed by future presidents might overturn segregation and oppose civil rights.

The Supreme Court reacted to criticisms by maintaining that the reason they struck down so many of the New Deal programs was not because the court was partisan, but because the Roosevelt Justice Department was badly unorganized and did not argue the cases well.

The furor surrounding the scheme appeared to make the Supreme Court back down possibly due to concerns that further conflict might lead to a constitutional crisis and the likely weakening of the judicial branch of government. The Wagner Act was upheld in April 1937 and the Social Security Act was declared constitutional in the following month.

Judicial Procedures Reform Bill of 1937 was dropped and never subject to a Senate Vote. The scheme damage the president's reputation and encouraged Conservative Democrats in Congress to work with Republicans to  oppose proposals for New Deal programs