What item represents one of the greatest sources of power for the President?

One of the greatest sources of presidential power is the legislative veto. A sitting president can prevent a law from being enacted by executing veto authority within 10 days of the passage of the law.

By simply threatening to veto legislation, a president wields power over Congress to have a bill amended before passage. Because a presidential veto is rarely overridden by Congress, the veto is a powerful tool that can be used for leverage during legislative battles. The president can use a regular veto that falls within the 10-day deadline or a pocket veto to reject a bill while Congress is officially adjourned.