Not long ago, a wild-eyed man came up to me in a large city, pushing a piece of paper into my hand and saying, in an alarmingly loud voice, "DO YOU KNOW WHERE THE IDEA OF THE UNITARY EXECUTIVE COMES FROM?" I couldn't help but laugh, because I do know (more or less), and the answer isn't quite what he said (which was Hitler, or it might have been Stalin). Because the idea of the unitary executive is so much in the news, and because it is creating a lot of confusion, it might be useful to set out some of the basics here.
Those who believe in a unitary executive need not think that the president can defy the will of Congress, or torture people, or make war on his own. The principle of a "unitary" executive involves only one thing: The president's hierarchical control over implementation ("execution") of federal law. Thus, for example, those who believe in a unitary executive tend to be drawn to the following sorts of positions:
1) as a matter of constitutional law, the president can fire all U.S. Attorneys at will;
2) as a matter of constitutional law, the president can fire the heads of executive agencies at will;
3) as a matter of constitutional law, the president has considerable control over policymaking by executive agencies (e.g., the EPA, the Department of Commerce, the Department of Labor), at least insofar as those agencies are acting within the limits set by Congress;
4) as a matter of constitutional law, the so-called independent regulatory commissions, such as the National Labor Relations Board and the Federal Trade Commission, are troublesome insofar as their heads are not at-will employees of the president;
5) any "independent counsel" statute is constitutionally troublesome insofar as the independent counsel is not an at-will employee of the president;
6) executive privilege can probably be extended, by the president, to all those who exercise discretion and are involved in law implementation within the executive branch.
In American constitutional law, the idea of a unitary executive is nothing new. It goes all the way back to the founding. The Constitution does not create a "plural" executive; Article II, section 1 vests executive power in one person, the president of the United States. The decision to create a unitary rather than plural executive was debated and decided. So in a way, everyone agrees that ours is a unitary executive. (Franklin Delano Roosevelt insisted on that point, and was especially dismayed when the Supreme Court ruled otherwise in its decision holding that the heads of the FTC are not the president's at-will employees.)
But there are real disputes, historical and otherwise, about what unitariness specifically involves. The general disagreement is between those who believe that ours is a "strongly unitary" executive, in the sense signalled by the six propositions listed above, and those who believe that the executive is only "weakly unitary," in the sense that Congress retains power (for example) to create independent agencies and independent prosecutors.
Gerhard Casper, for example, has vigorously argued that the Constitution gives Congress broad authority to structure the executive branch, by insulating law implementation from complete presidential control. Others, including Steve Calabresi, have vigorously disagreed, contending that the document and its history clearly forbid Congress from intruding on the president's authority to run the executive branch. (I am simplifying some complexities in their positions here.)
There are two dimensions to the debate over the nature of the unitary executive. The first is historical. What was the original understanding? The second is about interpretation across a span of many years. When the national government is so much larger than it originally was, is the claim for a strongly unitary executive more powerful, or less so, supposing that we seek to maintain fidelity to the document's original goals? Abner Greene has argued that the growth of the national government has made the argument for a strongly unitary executive branch less powerful; Larry Lessig and I have argued that the argument has become more powerful.
Under President Reagan, the executive branch argued, with real vigor, on behalf of a strongly unitary executive. The Supreme Court has resisted those arguments. But many issues are open, and in approaching the open questions recent presidents have tended to continue to argue that the executive is strongly unitary.
The most important point is that the claim for the unitary executive is not a general claim about the President's power to act on his own or to contradict the will of Congress. You can believe in a strongly unitary executive branch while also believing that the President cannot make war, or torture people, or engage in foreign surveillance without congressional authorization. You can also believe that the president can do a lot on his own, or a lot in violation of Congress' will, while also accepting the view that Congress can create independent agencies and independent prosecutors. In short, the debate over the unitary executive is an important but narrow one, and it is a small, distinct subpart of the general debate over presidential power.