Centralizing Border Control Policy Under the Supervision of the Attorney General, Letter Opinion for the Deputy Counsel to the President [March 20, 2002] [open pdf - 33KB]
"In general, the President may not transfer the functions of an agency statutorily created within one Cabinet department to another Cabinet department without an act of Congress. The President may not delegate his presidential authority to supervise and control the executive departments to a particular member of the Cabinet where no statutory authority exists to do so. The President may exercise his own power to establish a comprehensive border control policy for the federal Government and direct a single Cabinet member to lead and coordinate the efforts of all Cabinet agencies to implement that policy."
U.S. Dept. of Justice: http://www.justice.gov/