Constitutionality, or the ability to pass constitutional muster, is the test for laws and statutes to ensure that they do not violate the Constitution. Each law is believed to be constitutional, unless or until it is struck down in whole or in part. However, it has long been held that an unconstitutional law is void from its inception, not only after being declared so, and that no person is bound by it and no court or other agency is bound to enforce it.
When constitutionality is called into question, a case will be made that the law has caused damage to a person or entity. If the likelihood of damage can be shown, the person or entity is said to have standing to sue in order to attempt to have the law struck down as being unconstitutional. Any law or statute may be deemed unconstitutional as applied, which means as applied to the specific case, although it may retain its constitutionality in other situations. On the other hand, there are times when only certain sections of a statute are ruled unconstitutional and the remainder is allowed to stand. A law may also be ruled unconstitutional on its face, or fatally flawed, which means it is unconstitutional in its entirety.
There are different methods, also called levels of scrutiny, by which constitutionality is determined. Strict scrutiny is the most rigorous assessment and applies to First Amendment rights, suspect classes (i.e., classes based on race, national origin and sometimes alienage), and other fundamental rights (e.g., the right to vote, travel, privacy). When one of these classes or rights is violated and is defended in court, the government has the burden of proving that the disputed act or law is necessary to achieve a compelling government purpose.
Few challenged laws can pass constitutional muster under a strict scrutiny analysis, generally because at least one of the tests cannot be met. Even where the government can show a compelling interest, or persuasive reason for the law or statute in question, the remedy prescribed is often deemed too intrusive or restrictive. Therefore, in cases where strict scrutiny is applied, the government generally loses.
Intermediate scrutiny is the next strongest level of scrutiny. It's actually closer in rigor to strict scrutiny than to the lowest level of scrutiny. Intermediate scrutiny has been applied to cases involving gender discrimination, illegitimacy, and public education through the 12th grade for illegal aliens. The burden of proof is generally on the government to show that the disputed act or statute is substantially related to an important government purpose.
Finally, minimum scrutiny, more commonly referred to as rational basis, is somewhat open ended in that it provides that to uphold constitutionality the government must only show a permissible governmental goal achieved by reasonable means. Here the burden is on the challenger, not the government, who must show that the disputed act or statute is not rationally related to a legitimate government purpose. This test is applied to a variety of issues including cases involving poverty, wealth, age, and the necessities of life. Because this test is rather deferential to the government, the government usually wins when rational basis is applied.