Looks as if the issue is somewhat clouded, yet historically used with some self-limiting discretion by previous presidents.
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THE LEGAL SIGNIFICANCE OF PRESIDENTIAL
SIGNING STATEMENTS
Many Presidents have used signing statements to make substantive legal, constitutional, or administrative pronouncements on the bill being signed. Although the recent practice of issuing signing statements to create "legislative history" remains controversial, the other uses of Presidential signing statements generally serve legitimate and defensible purposes.November 3, 1993
MEMORANDUM FOR BERNARD N. NUSSBAUM,
COUNSEL TO THE PRESIDENT
This memorandum provides you with an analysis of the legal significance of Presidential signing statements. It is addressed to the questions that have been raised about the usefulness or validity of a such statements. We believe that such statements may on appropriate occasions perform useful and legally significant functions. These functions include (1) explaining to the public, and particularly to constituencies interested in the bill, what the President believes to be the likely effects of its adoption, (2) directing subordinate officers within the Executive Branch how to interpret or administer the enactment, and (3) informing Congress and the public that the Executive believes that a particular provision would be unconstitutional in certain of its applications, or that it is unconstitutional on its face, and that the provision will not be given effect by the Executive Branch to the extent that such enforcement would create an unconstitutional condition.(1)
These functions must be carefully distinguished from a much more controversial -- and apparently recent -- use of Presidential signing statements, i.e., to create legislative history to which the courts are expected to give some weight when construing the enactment. In what follows, we outline the rationales for the first three functions, and then consider arguments for and against the fourth function.(2) The Appendix to the memorandum surveys the use of signing statements by earlier Presidents and provides examples of such statements that were intended to have legal significance or effects.
http://www.usdoj.gov/olc/signing.htm