The United States is a constitution-based federal system. Federalism is a form of government in which power is distributed between a national (federal) government and local (state) governments.

The Constitution's Supremacy Clause states that federal laws, along with treaties and the Constitution itself, are the "supreme law of the land." However, it is clear that the Constitution created a federal government of limited powers. The Supreme Court has noted that every law enacted by Congress must be based on one or more of its powers listed, or enumerated, in the Constitution.

The Federal Government Has the Power To Declare War

These limited powers are set forth as what are termed "enumerated powers" in Article I, Section 8 of the Constitution. Among other things, these enumerated powers include: the power to levy taxes, regulate commerce, establish federal courts (underneath the Supreme Court), establish and maintain a military, and declare war.

In addition, the Supreme Court has interpreted the Constitution's Necessary and Proper Clause as laying out certain "implied powers." These are needed to carry out the principal powers enumerated in the Constitution. As Supreme Court Justice John Marshall noted in 1819, a government entrusted with great powers must also be entrusted with the power to execute them.

States Decide "Police Powers"

The Constitution thus grants broad powers to the federal government. However, these powers are limited by the 10th Amendment, which states that "the powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."

As James Madison explained, the powers given to each state "concern the lives, liberties, and properties of the people, and the internal order, improvement, and prosperity of the State."

These state powers are those required for public safety, health and welfare. They have generally been referred to as "police powers."

Sometimes, There Are Conflicting Laws

Finally, certain powers are called concurrent powers. These are powers the states and the federal government both may exercise concurrently, or at the same time. These can include, for example, setting up courts, levying taxes, and spending and borrowing money. Typically, these are powers necessary for the maintenance of public facilities.

At times it can be difficult to determine whether the federal government or a state government has the power to legislate on a particular matter. In general, the problem of conflicting laws between the states and the federal government has given rise to what is called the doctrine of preemption.

Under this doctrine, if a state or local law conflicts with a federal law, the state or local law must give way. As Justice Marshall put it in McCulloch v. Maryland, states do not have the power to slow or stop "or in any manner control the operations of the Constitutional laws enacted by Congress."

When Federal Law Overrides State Law

There are a variety of situations in which federal law preempts, or overrides, state law. Preemption occurs when there is an explicit, or clear and obvious, conflict between federal and state laws. It also occurs when it would be impossible for someone to follow, or comply with, both state and federal laws. Finally, it occurs when the purposes and objectives of the federal law would be thwarted by the state law.

The only exception to the doctrine of preemption is when a federal law is itself unconstitutional. In other words, a federal law is invalid if it exceeds the power of the federal government.