Correct answer: on the basis of the age of sitting judges.
Context/explanation:
President Franklin Delano Roosevelt (FDR) was eager to implement his New Deal programs as an antidote to the Great Depression. However, the US Supreme Court had already ruled that some provisions of the New Deal were unconstitutional, because they took too much power into the hands of the federal government, especially the executive branch of the federal government. So, riding the momentum of his landslide reelection victory in 1936, in February of 1937, FDR proposed a plan to expand the Supreme Court to as many as 15 judges. The plan offered to provide full pay to justices over age 70 who would retire. If the older justices didn't retire, assistant justices (with full voting rights) would be appointed to sit with those existing justices. This was a way FDR hoped to give the court a liberal majority that would side with his programs.
As it turned out, before FDR's proposal came up for a vote in Congress, two of the sitting justices came over to his side of the argument, and the Supreme Court narrowly approved as constitutional both the Social Security Act and the National Labor Relations Act. So his plan (which failed in the US Senate) became unnecessary to his purposes.
Roosevelt's "court-packing" scheme was unpopular. It was seen as an attempt to take away the independence of the judicial branch of government.
