Conservatives for the most part do not behave similarly (for whatever reason: perhaps in part a lack of opportunity). We have not sought to get the Supreme Court to command states to recognize fetal personhood, or to command public schools to organize prayers, or to forbid same-sex marriage. Our preference for formal constitutional amendments is itself a type of fidelity to the Constitution: fidelity, specifically, to Article V. We adhere to the form the Constitution itself lays down for how changes to it should be pursued.
The Constitution is a magnificent document, by and large, and embodies a great deal of wisdom about government. Consider, for example, the little-appreciated compact clause. The Founders acted on insights that public-choice theorists would spend much of the latter half of the 20th century reestablishing. They understood that state governments must sometimes cooperate, but also that their cooperative projects would often be sinister attempts to exploit other states or one another’s citizens. They understood as well that it would be impossible to spell out a rule in advance that authorized the good and necessary types of cooperation while barring the improper kind. So they established a rule: Any compact among the states requires the approval of a majority of Congress. It was an ingenious scheme; if it lacks anything, it is only a mechanism to make the Supreme Court enforce it. (The clause has been interpreted into nothingness.)
Conservatives sometimes say that it’s “the original Constitution” we favor, so as to distinguish it from the one that the Supreme Court has written. But this locution is misleading. Some of the amendments to the Constitution have added to its excellence or remedied its defects: above all, the Thirteenth Amendment, prohibiting slavery. But even the amendments that have less to recommend them are the law, and we are willing to abide by them until subsequent amendments nullify or modify them.
Constitutionalism, in short, is simply a special case of respect for the rule of law: the case in which the law in question is the supreme law of the land. The rule of law demands that those who apply the law — be they judges, sheriffs, presidents, or governors — apply it faithfully. If those officials can change the meaning of the words, there is no point to having a written law.
There are different types of originalism, and legitimate debate about what counts as originalism. But constitutionalism — which is, again, to say the rule of law — entails some sort of originalism. The law has to be knowable, and its meaning has to be fixed at the time of enactment (although, of course, its application may change based on the circumstances to which it applies). Officials who have to interpret the Constitution may or may not err in ascertaining the original meaning of the provisions at issue, or in inferring what norms that meaning implies, or in applying the norms to the legal case or policy dispute at hand. But any authoritative interpretation of the Constitution that departs from plausible understandings of the original meaning is itself a violation of the rule of law. Hence originalism is not merely one interpretive methodology among many.
To put it another way, constitutionalism rules out certain courses of action. It means that it is never acceptable for a congressman to vote for unconstitutional, or even doubtfully constitutional, legislation on the theory that the courts will sort it out. Nor can a constitutionalist judge treat the Constitution as simply raising questions — e.g., what is the right relation of society to the individual? — to which the judge makes up an authoritative answer. Both are cases of disobedience to the Constitution.
The fact that the Constitution is the law is not by itself sufficient reason to obey it. If it were radically unjust, we would have no such obligation; resistance to it, even on the part of officials exercising power under it, would in some cases be morally obligatory. Of course, and luckily, our Constitution is far better than that. But conservatives’ reverence for most of the content of our Constitution is the least important feature of constitutionalism, which is properly understood as the willingness to obey the Constitution, including those parts of it we do not revere.