Final answer:
The federal law requiring states to enforce a minimum legal age of 18 for tobacco purchases is the Synar Amendment. However, the legal age for such purchases has been raised to 21 years following recent federal changes.
Step-by-step explanation:
The federal law that requires states to set the minimum legal age for purchasing tobacco products at 18 years and mandates states to enforce this law is known as The Synar Amendment. This was part of the Alcohol, Drug Abuse, and Mental Health Administration Reorganization Act, focused on ensuring states complied with regulations to curb youth access to tobacco. Although initially set at 18, recent changes at the federal level have increased the minimum legal age for purchasing tobacco products to 21 years.
The federal government has also sought to influence states' policies on consumption-related behavior in other areas, such as the National Minimum Drinking Age Act of 1984, requiring states to raise their minimum drinking age to 21 or face a reduction in federal highway funds. This shows the federal government's ability to impact state laws by invoking fiscal incentives or penalties to promote compliance with national standards.