Q: Are presidential signing statements unconstitutional?
A: Probably not. But they don’t have much impact, either.
The use of presidential signing statements, which are official pronouncements made by the chief executive when signing a piece of legislation, dates back to the early 19th century. In more recent years, Ronald Reagan was an enthusiast and began making them on a regular basis. Subsequent presidents have followed his lead to a greater or lesser degree, with both Bill Clinton and George W. Bush being avid producers of these edicts.
Halstead, T.J. "Presidential Signing Statements: Constitutional and Institutional Implications." CRS Report for Congress, 17 Sept. 2007.
Savage, Charlie. "Examples of the President’s Signing Statements." The Boston Globe, 30 Apr. 2006.