Final answer:
Congress opposed Roosevelt's court-packing plan because it would increase the power of the executive branch by allowing him to appoint additional justices, thereby undermining the judiciary's independence.
Step-by-step explanation:
Many members of Congress opposed President Roosevelt's plan to pack the courts primarily because it threatened to increase the power of the executive branch. This was seen as an indirect method for Roosevelt to ensure compliance of the judiciary with his New Deal legislation, after facing several adverse Supreme Court decisions that declared parts of the New Deal unconstitutional. His judicial reform measure, framed as a way to make the court more efficient, was perceived as a power grab due to the proposed addition of new justices sympathetic to his policies.
Roosevelt's action was dubbed the 'court-packing scheme,' and proposed that additional judges would be appointed to serve alongside any federal judge over the age of seventy. This move would allow Roosevelt to significantly influence the judiciary - an unprecedented change that sparked significant backlash and division, even among Roosevelt's supporters and leading Democrats. Eventually, the plan failed to gain the necessary support in Congress, and Roosevelt had to retreat from his proposition.
The key concerns centered on the dangers of upsetting the delicate balance of power among the governmental branches and the potential to undermine the judiciary's role as a check on executive power.