Article VI, clause 3 of the U.S. Constitution reads

The Senators and Representatives before mentioned...shall be bound by Oath or Affirmation, to support this Constitution...

Accordingly, on January 3rd, members of the 113th U.S. Congress took the following oath

I do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter.

Who might be domestic enemies of our Constitution?

In previous American Thinker posts, I have written about the wisdom of the late Henry Hazlitt.  As to the question posed above, Hazlitt articulated well the answer in a 1956 article and other writings. 

"The greatest threat to American liberty today," he wrote, "comes from within."  Specifically, Hazlitt was referring to "a growing and spreading totalitarian ideology."  Uncomfortable though it may be to say or express it, proponents of that ideology were then and are now the enemy referenced in the congressional oath.  They are those who are hostile to our heritage.

"[It] isn't too difficult to recognize the totalitarian mind," and by implication the devotees of the doctrine of government control over the individual, "when we meet one."  In short "Its outstanding mark is a contempt for liberty."  Acknowledging the difficulty in precisely defining liberty, Hazlitt contrasted it with its antithesis, slavery.

The roots of totalitarianism lie in the "contemporary faith in the necessity and benevolence of a continually expanding government intervention."  Totalitarians, according to Hazlitt, want total control, but not necessarily total suppression.  They "suppress merely the ideas which they don't agree with, or of which they are suspicious, or of which they have never heard before; and they suppress only the actions that they don't like, or of which they cannot see the necessity. They leave the individual perfectly free to agree with them, and perfectly free to act in any way that serves their purposes..." 

Hazlitt prophetically described "three main tendencies or tenets" toward the "road to totalitarianism" that we find ourselves on.  First among them is "the tendency of the government to attempt more and more to intervene, and to control economic life," that is, the

...pressure for a constant increase in governmental powers, for a constant widening of the governmental sphere of intervention. It is the tendency toward more and more regulation of every sphere of economic life, toward more and more restriction of the liberties of the individual. The tendency toward more and more governmental spending is a part of this trend. It means in effect that the individual is able to spend less and less of the income he earns on the things he himself wants, while the government takes more and more of his income from him to spend it in the ways that it thinks wise. One of the basic assumptions of totalitarianism, in brief (and of such steps toward it as socialism, state paternalism, and Keynesianism), is that the citizen cannot be trusted to spend his own money. As government control becomes wider and wider, individual discretion, the individual's control of his own affairs in all directions, necessarily becomes narrower and narrower. In sum, liberty is constantly diminished.

The second main step to totalitarianism is, according to Hazlitt, "the tendency toward greater and greater concentration of power in the central government at the expense of local governments," that is,

...the growth of power in Washington at the expense of the states.

The concentration of power and the centralization of power...are merely two names for the same thing. This second tendency is a necessary consequence of the first. If the central government is to control more and more of our economic life, it cannot permit this to be done by the individual states. The pressure for uniformity, and the pressure for centralization of power, are two aspects of the same pressure.

...Planning from the center is possible only with centralization of governmental power...[The] federal government assumes more and more of the powers previously exercised by the states, or powers never exercised by any state; and the Supreme Court keeps steadily stretching the interstate commerce clause of the Constitution to authorize powers and federal interventions never dreamed of by the Founding Fathers. At the same time recent Supreme Court decisions treat the Tenth Amendment to the Constitution practically as if it did not exist.

The third step is "the tendency toward more and more concentration of power in the hands of the executive at the expense of the legislative and judiciary." According to Hazlitt,

In the United States this tendency is very marked today. To listen to our pro-totalitarians, the main duty of Congress is to follow the president's "leadership" in all things; to be a set of yes-men; to act as a mere rubber-stamp.

The dangers of one-man rule have been so emphasized and dramatized in recent years...that any warning of this danger to Americans may seem needless. Yet most Americans, like the citizens of the countries already victimized by their native [totalitarians], may prove incapable of recognizing this evil until it has grown beyond the point of control.  One invariable accompaniment of the growth of Caesarism is the growing contempt expressed for legislative bodies, and impatience with their "dilatoriness" in enacting the "Leader's" program, or their actual "obstructionist tactics" or "crippling amendments." Yet in recent years derision of Congress has become in America almost a national pastime. And a substantial part of the press never tires of reviling Congress for "doing nothing" - that is, for not piling more mountains of legislation on the existing mountains of legislation - or for failing to enact in full "the President's program.

What invariably results is capitulation and an ambiguous law "setting forth a number of vague but high-sounding goals and [creation of another] agency or commission" that "proceeds to become a prosecutor, court, and legislative body all rolled into one" and "starts laying down a series of rulings and handing down a series of decisions, many of which surprise no one more than the congressional members who created the agency in the first place."

Hazlitt had much more to convey about the dictatorial trend, the enemy within, and their tactics and techniques.  I recommend reading the entire article and other of his books and publications.  We are far down the road Hazlitt warned us about -- due in large part to U.S. Senators' and Representatives' forsaking of their sworn duty to "support and defend the Constitution of the United States against all enemies, foreign and domestic."