It is, as well, a barrier to excesses by that government, a barrier that operates at two levels. It reserves unenumerated powers to the States and the to the People. It provides so-called "checks and balances" between the parts of the federal government.
But the Constitution is no more the government than a blue print is a house.
But if a house is to be built, the blueprint must be honored. By honored, I mean only that the craftsmen follow the measures and designs, the construction materials lists, and assemble the product as proposed in the elevations and drawings.
If the federal government envisioned by the Framers and Ratifiers of the Constitution is to come into existence, then it too must be honored by the craftsmen that follow.
The difference here is that the craftsmen are office holders and their lawful designees, individuals that alone or by groups wield powers that are categorized as either legislative (the making of law), executive (the enforcing of law), or judicial (the evaluation of the enforcement of law). What has actually happened with our national government, however, has been the governmental equivalent of carpenters, masons, plumbers, and electricians "ad libbing" their efforts rather than following the blue print.
Some examples illustrate my meaning.
Take Thomas Jefferson, whose basic philosophy was very like that of Antonin Scalia. He viewed the security of the liberties of the People as dependent on a Constitution of fixed, determined meaning, what Scalia referred to as the "hide-bound" Constitution. Still, the Chief Executive of a young Nation, Jefferson sees a swath of desirable territory that lately is available. Now the expansion of the national boundaries of the nation, drawing within its border the considerable natural resources there located, is a good. But it is a good the accomplishment of which, at a minimum, required the advise and consent of the Senate (as to a treaty for purchase), and the appropriation of necessary funds by the entire Congress. Jefferson follows the age old adage that it is better to ask forgiveness than permission and negotiates the Louisiana Purchase.
Or take the Federalists. Upon ratification of the Constitution and the standing up of the national government, the Federalists had the Congress and the White House. They had populated the new judicial system with federalist judges. They sought to coalesce their control of the country by limiting dissent. To do so, they enacted the Alien and Sedition Acts. These acts clearly violated the prohibition stated in the First Amendment that Congress shall make no law abridging the Freedom of Speech. Yet they did just that.
Military adventurism without a formal declaration of war, in my lifetime includes Vietnam, Grenada, Panama, among others. Others went before. There is NO CONSTITUTIONAL JUSTIFICATION for a President to conduct war against another nation in the absence of a declaration of war, yet it is done all the time.
For that matter, the myriad, hundreds of thousands, of pages of federal regulations reflect Congress allowing executive branch agencies to impose regulations like the Executive Branch had law making authority. Moreover, the enforcement mechanisms by which such agencies conduct hearing on rule violations smack of judicial adventurism by the Executive.
Ultimately, there is this terrible, hopeful monster of excursions, excesses, and entropies, by which we are now MASTERED.
You may not think about these things much. When I think about them, I sometimes wonder if things are now as they are because too few anymore care to know the meaning of the Constitution or to discipline the agents of the government for their violations of it.