Most people often mistakenly refer to our nation as the greatest democracy on earth. They are mistaken because we are not an absolute democracy; we are a constitutional republic. That is what makes our nation great, for if we were merely a democracy, we would be anything but great. And to the extent that we no longer function as a constitutional republic, that greatness is rapidly ebbing away.
Why did we need a constitution? Why are popular elections not a sufficient means of preserving liberty?
A pure unbridled democracy is a political system in which the majority enjoys absolute power by means of democratic elections. In an unvarnished democracy, unrestrained by a constitution, the majority can vote to impose tyranny on themselves and the minority opposition. They can vote to elect those who will infringe upon our inalienable God-given rights. Thomas Jefferson referred to this as elected despotism in Notes on the State of Virginia (also cited in Federalist 48 by Madison):
An ELECTIVE DESPOTISM was not the government we fought for; but one which should not only be founded on free principles, but in which the powers of government should be so divided and balanced among several bodies of magistracy, as that no one could transcend their legal limits, without being effectually checked and restrained by the others.
Thus, a constitution that limited and divided the power of government was necessary to preclude elected officials from imposing tyranny on the people. This is why they adopted a constitution with limited enumerated power, divided and checked across several branches and levels.