U.S.: Balance of Power
The Supremacy clause may be used by courts if the decision needs to be made on if a state law is invalid because it contradicts federal law. The courts must look at the two laws and how they are interpreted and used and then decide if there is a conflict between the state and federal law. There is an existing problem if the state law would stop or cause some problem with the completion of the full plan of congress.
There are several distinctions between the federal, state, and local governments. The authority of the federal government to regulate business activity is from what is stated in the commerce clause. The state and local government authority comes from something called police powers. The police powers are said to require state law and to protect the people and their safety, health, and general welfare. This gives the state more power to regulate business activities. Some areas of government regulation is known to be completely federal or completely local. If a subject area is exclusively federal, then a state law of the same subject area can be void, even if congress has not passed a law in that area yet. It is becoming more difficult to find a subject area that is completely and exclusively local. When there is not federal regulation of a subject then it is possible that state regulation will be allowed.