Chief Law Enforcement Officer. The President has the sole constitutional obligation to "take care that the laws be faithfully executed,"17 and this grants him broad discretion over federal law enforcement decisions. He has not only the power, but also the responsibility to see that the Constitution and laws are interpreted correctly.18 In addition, the President has absolute prosecutorial discretion in declining to bring criminal indictments. As in the exercise of any other constitutional power, one may argue that a particular President is "abusing his discretion," but even in such a case, he cannot be compelled to prosecute any criminal charges.
Head of the Executive Branch. The Framers debated and rejected the creation of a plural executive. They selected a "unitary executive" and determined that he alone would be vested with "[t]he executive power" of Article II. After much debate, the Framers also determined that the President would nominate and appoint (with the Senate's consent in some cases) all officers in the executive branch. With very few exceptions, all appointed officials who work in the executive branch serve at the will and pleasure of the President, even if Congress has specified a term of years for a particular office.19 All of this was designed to ensure the President's control over officials in the executive branch20 and to promote "energy in the executive."21
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When the President is lawfully exercising one of these functions,22 the scope of his power to issue written directives is exceedingly broad. In short, he may issue or execute whatever written directives, orders, guidelines (such as prosecutorial guidelines or nondiscriminatory enforcement policies), communiqués, dispatches, or other instructions he deems appropriate.
The President also may issue directives in the exercise of his statutorily delegated authority, unless Congress has specified in law that the statutory power may be exercised only in a particular way. A few examples of Congress's conditional grant of statutory authority are mentioned herein, but as previously explained, there are limits to how far Congress can go in an attempt to micromanage even the President's statutorily delegated authority.23 For example, Congress can grant the President (or his Attorney General) the authority to deport certain illegal aliens, but it cannot attempt to retain a veto over the final decision as it tried to do in the Immigration and Nationality Act.24
In sum, a President has broad discretion to use written directives when he is lawfully exercising one of his constitutional or statutorily delegated powers. Any broad power or discretion can be abused, but it would be wrong to confuse such potential or real abuse with the many legitimate uses.
http://www.heritage.org/research/reports/2001/02/the-use-and-abuse-of-executive-orders-and-other-presidential-directives