The unitary executive doctrine arises out of a theory called "departmentalism," or "coordinate construction." According to legal scholars Christopher Yoo, Steven Calabresi, and Anthony Colangelo, the coordinate construction approach "holds that all three branches of the federal government have the power and duty to interpret the Constitution." According to this theory, the president may (and indeed, must) interpret laws, equally as much as the courts.
The Unitary Executive Versus Judicial SupremacyThe coordinate construction theory counters the long-standing notion of "judicial supremacy," articulated by Supreme Court Chief Justice John Marshall in 1803, in the famous case of Marbury v. Madison, which held that the Court is the final arbiter of what is and is not the law. Marshall famously wrote there: "It is emphatically the province and duty of the judicial department to say what the law is."
-snip-Of course, the President has a duty not to undermine his own office, as University of Miami law professor A. Michael Froomkin notes. And, as Kelley points out, the President is bound by his oath of office and the "Take Care clause" to preserve, protect, and defend the Constitution and to "take care" that the laws are faithfully executed. And those duties require, in turn, that the President interpret what is, and is not constitutional, at least when overseeing the actions of executive agencies.
However, Bush's recent actions make it clear that he interprets the coordinate construction approach extremely aggressively. In his view, and the view of his Administration, that doctrine gives him license to overrule and bypass Congress or the courts, based on his own interpretations of the Constitution -- even where that violates long-established laws and treaties, counters recent legislation that he has himself signed, or (as shown by recent developments in the Padilla case) involves offering a federal court contradictory justifications for a detention.
This is a form of presidential rebellion against Congress and the courts, and possibly a violation of President Bush's oath of office, as well.
http://writ.news.findlaw.com/commentary/20060109_bergen.html