Final answer:
Congress refused to enact President Franklin D. Roosevelt's court-packing plan because it threatened to upset the constitutional system of checks and balances.
Step-by-step explanation:
Congress refused to enact President Franklin D. Roosevelt's court-packing plan because the plan threatened to upset the constitutional system of checks and balances. Roosevelt's proposal to add up to six additional judges to the Supreme Court would have increased the size of the Court and given him the power to appoint judges who supported his views. This would have undermined the independence of the judiciary and upset the balance of power between the three branches of government.