Final answer:
Roosevelt's 'court-packing plan' was denied by Congress because it threatened the principle of checks and balances, aiming to increase the Supreme Court's justices and potentially shift its rulings in favor of the New Deal.
Step-by-step explanation:
President Franklin D. Roosevelt's plan to increase the number of justices on the Supreme Court was ultimately rejected by Congress. The main concern regarding Roosevelt's proposal, known as the "court-packing plan," was that it threatened the principle of checks and balances. The plan would have permitted the President to appoint an additional Supreme Court justice for each sitting justice over the age of seventy, with the potential to add up to six new justices, thereby expanding the court from nine to possibly fifteen members. This was seen as an attempt to sway the court in favor of Roosevelt’s New Deal legislation, which had faced several legal challenges and had a number of components struck down by the Court as unconstitutional, such as the National Recovery Act (NRA) and the Agricultural Adjustment Act (AAA). Ultimately, the plan was viewed as an executive overreach that could undermine the independent judiciary, leading to Roosevelt's setback in this controversial proposal.