Line-Item Veto Quote
There is no provision in the Constitution that authorizes the President to enact, to amend, or to repeal statutes. . . .There are important differences between the President’s “return” of a bill pursuant to Article I, §7, and the exercise of the President’s cancellation authority pursuant to the Line Item Veto Act. The constitutional return takes place before the bill becomes law; the statutory cancellation occurs after the bill becomes law. The constitutional return is of the entire bill; the statutory cancellation is of only a part. Although the Constitution expressly authorizes the President to play a role in the process of enacting statutes, it is silent on the subject of unilateral Presidential action that either repeals or amends parts of duly enacted statutes.
There are powerful reasons for construing constitutional silence on this profoundly important issue as equivalent to an express prohibition.
Clinton v. City of New York, 524 U.S. 417 (1998)