Final answer:
The concept of states having the ultimate authority over national laws is known as nullification. The Supremacy Clause and historic rulings have, however, established national supremacy where federal laws prevail in conflicts with state laws.
Step-by-step explanation:
The claim that states should have ultimate authority to determine the welfare of citizens and can nullify national laws is referred to as nullification. This concept has been part of an ongoing debate about the balance of power between the state and federal governments. The Supremacy Clause of the Constitution, as well as historical events such as the Civil War and landmark Supreme Court cases like McCulloch v. Maryland, have reinforced the principle of national supremacy, indicating that legitimate national laws take precedence over conflicting state laws. Despite these rulings, discussions about the extent of state power continue, particularly with state-specific policies and when states challenge the scope of federal authority.