Why can a law itself be challenged under substantive due process?

A law can be challenged under substantive due process because the content of the legislation itself may be considered repugnant to the Constitution, regardless of how fairly or regularly the law is enforced.1

While procedural due process focuses on the rules and methods of enforcement, substantive due process addresses the possibility that a law may inherently violate fundamental rights. This legal doctrine is based on the principle that certain rights are so deeply rooted in the traditions and conscience of the people that they are considered "implicit in the concept of ordered liberty".12

Consequently, if a law is deemed to unfairly, arbitrarily, or unreasonably deprive an individual of life, liberty, or property, it may be challenged. The Supreme Court evaluates such challenges by determining whether the law reasonably promotes the public welfare and whether the means selected bear a reasonable relationship to a legitimate public interest. If a law regulates a fundamental right, it must meet a stricter standard of judicial scrutiny, known as the compelling interest test.312