What is preemption and why Is it often unpopular with advocates of the state's rights?

Conflict preemption compliance with both federal and state regulations is a physical impossibility, or, where state law stands as an obstacle to the accomplishment and execution of the full purposes and objectives of congress.

When state law and federal law conflict, federal law displaces, or preempts, state law, due to the Supremacy Clause of the Constitution. U. S. Const. art. VI., § 2. Preemption applies regardless of whether the conflicting laws come from legislatures, courts, administrative agencies, or constitutions. For example, the Voting Rights Act, an act of Congress, preempts state constitutions, and FDA regulations may preempt state court judgments in cases involving prescription drugs.

Congress has preempted state regulation in many areas. In some cases, such as medical devices, Congress preempted all state regulation. In others, such as labels on prescription drugs, Congress allowed federal regulatory agencies to set national minimum standards, but did not preempt state regulations imposing more stringent standards than those imposed by federal regulators. Where rules or regulations do not clearly state whether or not preemption should apply, the Supreme Court tries to follow lawmakers' intent, and prefers interpretations that avoid preempting state laws.