Separation of Powers:
The Court argued that the line-item veto allowed the President to unilaterally amend or repeal portions of statutes, which was a power not granted to the President by the Constitution.
Presentment Clause:
The President must either approve the entire bill or reject it, not selectively cancel portions of it.
No constitutional amendment:
The Court emphasized that Congress could not alter the fundamental principles of the lawmaking process (as outlined in the Presentment Clause) without a constitutional amendment.