Helvering v. Davis, 301 U.S. 619 (1937), was a decision by the United States Supreme Court, which held that Social Security was constitutionally permissible as an exercise of the federal power to spend for the general welfare, and did not contravene the 10th Amendment. The Court’s 7–2 decision defended the constitutionality of the Social Security Act of 1935, requiring only that welfare spending be for the common benefit as distinguished from some mere local purpose. It affirmed a District Court decree that held that the tax upon employees was not properly at issue, and that the tax upon employers was constitutional.
Helvering upheld the constitutionality of Social Security on the basis that Congress has a general power to spend on whatever it deems to be in the general welfare.
This ruling completely upended the system of enumerated powers, in which Congress only had the powers delegated to it by the Constitution, and eviscerated the Tenth Amendment that restricted the federal government to its defined roles.
Since Helvering, Congress can spend money on anything it wants, facilitating the welfare state and the immense growth of the federal government in the last 80 years. If I had to make a rough estimate, I’d say about 75% or more of the spending currently done by the federal government relies on this holding in Helvering, making the overwhelming majority of what the federal government does unconstitutional.
Thus, Helvering is the central case that flipped the system from limiting the government to what is explicitly allowed to permitting anything that isn’t explicitly banned — effectively ending federalism.