A conventional narrative of constitutional history says that in the late 19th and early 20th centuries, the U.S. Supreme Court commonly used the “due process” clause of the Fifth and 14th amendments to strike down laws, particularly in the economic realm, under a theory of substantive due process.
The 14th Amendment version of the clause provides: “nor shall any state deprive any person of life, liberty, or property, without due process of law.”
The language would seem to clearly limit only the procedure (“process”) government uses when depriving citizens of life, liberty or property (i.e., through capital punishment, imprisonment and fines). That sense is strengthened by its placement in the Fifth Amendment which addresses other matters of criminal procedure (though not exclusively). A rough description of the substantive due process theory is that there may be some government deprivations that are unconstitutional regardless of the procedure followed because they cannot be justified by any appropriate public purpose.
Again, the conventional narrative is that a hidebound Supreme Court used this constitutionally shaky theory to insert itself into economic regulations until the crisis of the Great Depression caused it to be discredited and largely abandoned with court majorities switching to defer to New Deal regulations.
So, the doctrine of substantive due process is now supposed to be out of favor.
The temptation to read policy preferences into the Constitution is strong, however. The focus appears to have turned to another clause in the 14th Amendment: “nor deny to any person within its jurisdiction the equal protection of the laws.”
Those who have followed the arguments in the U.S. Supreme Court over the definition of marriage know that advocates of redefining marriage are primarily arguing that current marriage laws violate the Equal Protection Clause. They would like the courts to adopt a substantive equal protection theory.
There are two alternative routes they are urging.
The first is for the courts to say that “sexual orientation” is analogous to race (building on the gloss of a footnote in a 1938 case), so the court should put its thumb on the scale in weighing laws that affect this group.
The second is to say that the law has no “rational basis” for treating the group differently. The Supreme Court has decided less than a dozen cases on these grounds, including a case relied on heavily by the same-sex marriage advocates in which the court said the government had to give food stamps to households consisting of unrelated people.
Either way, the courts would be using the equal protection clause the way the due process clause used to be used by some judges, to provide “substantive” rights not even contemplated by the Framers of the Constitution’s provisions. Central to our system is a written Constitution. The very nature of such a document means that its words must be taken seriously on their own terms and not become merely empty vessels into which judicial preferences are poured.