Final answer:
The main argument against President Franklin D. Roosevelt's plan to increase the number of Supreme Court justices was that it gave the executive branch too much power, disrupting the balance among the branches of government.
Step-by-step explanation:
President Franklin D. Roosevelt's plan to increase the number of justices on the Supreme Court, often referred to as the Supreme Court Packing Plan, faced significant opposition. The main argument against this plan was that it gives the executive branch too much power, infringing on the system of checks and balances designed to prevent any one branch of government from becoming too dominant. Roosevelt's intention behind the Court-Packing Scheme was seen as a way to influence the judiciary by appointing additional justices who would be sympathetic to his New Deal policies, which the Supreme Court had previously deemed unconstitutional in part because they vested too much authority in the federal government.
After his landslide reelection, Roosevelt misread the public's support as a blank check to expand executive influence, rather than a rejection of the Republican alternative. While his aim was to ensure a more New Deal-favorable Supreme Court, the plan sparked fears among the public and politicians that the president was overreaching. Critics painted the move as undemocratic, undermining the Court's independence, and setting a dangerous precedent for future executives seeking to circumvent constitutional constraints.