It is true that the Constitution is over two hundred years old and that much has changed in the way we use the English language, what we understand of logic, syntax, tropes, signs and signifiers. Indeed much has changed in the way we view life. We now understand the germ (and virus) theory of disease, which our Founding Fathers did not. We now know that life is built using a DNA template, which also can be faulty and lead to disabilities and predispositions for disease. Our grandparents had not a clue about this, let alone the Founding Fathers. We now understand electricity and electromagnetic radiation, whereas the Founding Fathers had only very crude understanding of electricity and knew nothing of EMR, radio, television, cell phones, radar, microwave ovens, etc. We understand semi-conductors and have computer and internet systems that fulfill the dreams of the ancient encyclopedists, none of which was even vaguely contemplated in the last decades before the industrial revolution in America. We have discovered a replacement for whale oil and, in fact, have exploited the planet for it. We have mechanized society to an extent never imagined by virtually anyone of the generation of our Founding Fathers, including Franklin and Jefferson (or even ourselves)! America is a far, far different place and Americans are a much more diverse and much larger group of people now than in 1789. (Population: 2006 --296 million; 1790 --about 12 million of which 4 million were white males eligible to vote.)
A scant half generation later in 1803 things were not much different when Supreme Court Justice John Marshall wrote his opinion in Marbury v. Madison firmly establishing the principle of Judicial Supremacy in the Judicial Branch of government and, of course, that the Supreme Court is the final arbiter of the Constitutionality of Legislation.
Just for a moment let's look at the document and see how ambiguous it is or is not on the idea of separation of powers. It does not literally say that the government of the United States shall be constructed to strictly separate the three principal functions of government, but of course these matters were discussed and discussed again and again. The Constitution was written in the environment of this discussion and reflects that very principal. Article I, Section 1 is pretty explicit. It says "All legislative powers herein granted shall be vested in a Congress of the United States ...." There is nothing ambiguous about that word "all." It does not say that the President can rewrite or line-item veto any part of legislation. It says that legislation comes from the Legislative Branch exclusively. All means ALL.
The Unitary Executive theory is based on the notion that the President is required to know the law and the Constitution in order that "...he shall take care that the laws be faithfully executed, ..." (Article III, Section 3). This simply means that the President is charged with conforming to the Constitution in the carrying out of duly legislated laws. It means he is not allowed to violate the Constitution while attempting to carry out a law. In other words, for instance, protecting us from hypothetical or real terrorists does not confer the right to ignore the Fourth Amendment.
Moreover, in the prescribed oath of office (Article II, Section 1, last clause) the President is required to the best of his ability to "preserve, protect and defend the Constitution of the United States." In fact, Bush has interpreted his oath and Article II, Section 3 to mean that federal Executive is Constitutionally required to carry out its own legislative and judicial processes. In this theory the Executive is "unitary" because it combines all three functions of government into one swell package.
George W. Bush (and his mentors Dick Cheney and Donald Rumsfeld) took to the theory of the Unitary Executive like Marine Corps landing craft to a hostile beachhead. To give you an idea of the tautologically absurdity of the notion, though, consider that George has relied on the analysis not of arm's-length objective thinkers but instead hired-gun attorneys, whose very livelihoods depended upon pleasing George and his junta. These three "scholars" have usurped the function of the courts and done so on the connivance of the President. It is, whether they like to think of it that way or not, a criminal conspiracy ... and they must be brought to Justice.
We learn of late that George has taken upon himself the privilege of interpreting and altering legislation by issuing "signing statements" that may reverse or negate the intent of the legislation. These statements would be unnecessary, of course, if his own party agreed with him. But that is not the case. The Congress can over-ride any Presidential veto, so, for example, in the case of the legislatively mandated banning of torture, George usurped the legislative branch by issuing a contradictory signing statement.
Obviously the "Unitary Executive" theory is dangerous nonsense. Just as obviously, though, is the fact that since Richard Milhous Nixon the notion has been gaining some popularity inside the Con Law profession, among those who think that the changed circumstances of our era demand changed views about our form of government. Okay, let us suppose for a hypothetical moment that modern nuclear power and weapons, anti-biotics, television, computers, internet, interstate highways, global corporations, extended life expectancy, and maybe a hundred other significant changes since 1789 have rendered our the separation of powers doctrine built into our Constitution obsolescent. How would one go about changing the system to something more suitable?
Given that the voters and citizens of the United States want to continue to have some say in the governing of the nation and themselves, that in fact democracy is both the means and the end of our preferred system, then would a slow-motion putsch be the way to accomplish the necessary change? Well of course not! But who among us can recall that Bush said "a vote for me is a vote for a new system of American government?" Who can recall any talk of the Unitary Executive during the 2000 or 2004 campaigns? No one can, of course; it did not happen. So, we are left with one ugly fact and at least one very unpleasant hypothesis.
The incrementalism of the revolution is a brilliant strategy. It takes into account the persistent indifference of Americans to liberty:
... to most Europeans the United States is still regarded naively as the land of liberty par excellence, whereas to most Americans the thing itself has long ceased to have any significance, and to large numbers of them, indeed, it has of late taken on an extreme obnoxiousness. I know of no civilized country, indeed, in which liberty is less esteemed than it is in the United States today; certainly there is none in which more persistent efforts are made to limit it and pull it down.
H.L.Mencken, "H.L.Mencken" in The Nation, 12/5/1923.
It takes into account the acknowledged attention span of the body politic and its servants in the Fourth Estate, and it relies upon the willingness of Congressional members of the Republican Party in Congress to violate and ignore their oaths of office ... partly out of fear (of Rove and Cheney), partly out of greed and corruption!