I mention the Equal Protection clause specifically because of one of the other essential properties of all republics: a large degree of equality among the citizens of the republic, both in terms of protections and duties.
As Thomas Jefferson--writing in his Commonplace Book--states, "[As Montesquieu wrote in Spirit of the Laws, [Book] VI, c[hapter] 2:] 'In republican governments, men are all equal; equal they are also in despotic governments: in the former because they are everything; in the latter because they are nothing.'"
I also believe that there are implicit duties placed upon all citizens by our Constitution. A primary example of these is our duty to serve on a jury if we are called. If we have a right to a trial by a jury of our peers under the Sixth Amendment, then it is logical that we have a responsibility to be a member of a jury should one of our peers be placed on trial.
All of these duties and other limitations are necessary to the proper functioning of a republican form of government. While the lex majoris partis (majority rule) may be at the heart of any republican government, our Founders and Framers--avid readers of the histories of Athens and Rome--understood that unscrupulous demagogues could take advantage of a tumultuous situation and, scapegoating some segment of society, impose a tyranny against them. As James Madison so wisely discerned, a republic requires checks and balances; not only against the government itself, but against any potential tyranny imposed by either a majority or a minority of its citizens.
Laissez-faire capitalism completely lacks any sort of duties, checks or balances.
I can hear all of you free market disciples asking, "Well, what about Adam Smith's Invisible Hand of competition?"
The problem is that unfettered, laissez-faire capitalism, by its very nature, does everything that it can to eliminate competition, as a means to maximize profits.
It is self-evident that it is in the best interest of the laissez-faire capitalist to do anything they can--including eliminating the competition--to increase their business's profitability. To quote Kenneth Lux, "The saving grace was supposed to be the "invisible hand" of competition"[C]ompetition would keep these instincts [to drive competitors out of business] and "expensive vanities"...in line. Smith would hardly have been surprised at the motives of Rockefeller, but"would have been chagrined at his success. Smith"overlooked the possibility that self-interest would work to undermine and eliminate competition and"tie up the invisible hand. It is"unrestrained self-interest that is the fundamental flaw in any absolute policy of laissez-faire." (Adam Smith's Mistake: How a Moral Philosopher Invented Economics and Ended Morality; 1990, p.p. 118-9.)
The Founders and Framers of the United States of America and its Constitution understood the need for social and political checks and balances to insure the proper functioning of their new republic. However, the study of Economics was in its infancy: Smith's The Wealth of Nations was published the same year that Thomas Jefferson wrote the Declaration of Independence.
Only the two most prescient among them, Thomas Jefferson and James Madison, recognized both that economics was the third leg of the stool upon which the new American Republic sat, and the need for a system of checks and balances in this newly recognized dimension of a modern society. These two Virginia sages did everything they could to add an Amendment to the Bill of Rights that would have limited the power and scope of corporations in the landscape of America, but failed against the opposition of Alexander Hamilton and the Federalists.
Over the last two centuries, the fears of Jefferson and Madison have proven to be well founded, as corporations have risen from a role as servants of the public good, to near masters of our body politic. As diverse group of individuals as Martin Van Buren, Abraham Lincoln, Senator John Sherman, Theodore Roosevelt, Eugene V. Debs, Franklin Roosevelt, and Ralph Nader have warned us against the burgeoning power of corporations in America. The danger inherent in this power increased many times over when in 1886--due to a mistaken interpretation of a Clerk's preface in the decision of Santa Clara County v. Southern Pacific Railroad-- the Supreme Court granted corporations the privilege of the same rights as individuals under the Fourteenth Amendment's Equal Protection Clause. (See Thom Hartmann's book Unequal Protection for more on this travesty in American History, and its deleterious effect on our country, its people, and its economy.)
I believe that Ambrose Bierce nailed the real problem in his book The Devil's Dictionary. "Corporation. An ingenious device for obtaining individual profit without individual responsibility."