Presidents have a veto power called the Signing Statement. The statement is a document that explains why the president finds a certain portion of a bill unconstitutional and/or why it will not be enforced by that president's administration. Other parts of the bill may be enforced, so it's sort of a line item veto in that way.
With that in mind, I have wondered about the historic weakness of the executive branch (except in a few notable cases) and I am not displeased that the Bush Administration used the Signing Statement over 1,000 times. Recall, for example, the fundamentals of our government, the separation of powers, and checks and balances of each branch. The courts pass judgment on legislation, the legislatures pass laws enforced by the executive, and the executive - well, he can veto laws - but mostly he does what the legislature and the courts tell him to do.
That's a weak position not representative of an equal branch. In fact, a strong, or equal executive branch should have push-back equivalent to any other branch. A president should indeed be able to declare a bill unconstitutional - every other branch can.
The redundancy of power among the branches is apparent. Congressional hearings look like trials and have been known to render judgments that resulted in financial penalties and incarceration. And who hasn't heard about judges legislating from the bench, famously finding, for example, a right to privacy in the Constitution where none exists. The executive's attorney general can take opponents and wrongdoers to court - which is tantamount to controlling the court, if not the outcome.
When Lincoln assumed all powers not elsewhere delegated in the constitution and then suspended habeas corpus he provoked an historic battle between the Presidency and the Supreme Court. Lincoln, as Commander in Chief, refused to turn over a secessionist when the Supreme Court ruled that he must. He defied the court based on his interpretation of the constitution and the president's war powers.
Would that President G.W. Bush had the same strength. We would have delighted in his imprisoning any congressmen or federal judges during his tenure. He certainly had ample cause. Sadly, despite all the laws and all the power and all the authority resident there, there appears to be no penalty for either wanton misbehavior or abject ineptitude in Washington.