The President's powers are limited due to checks and balances. The President, for example, cannot evaluate the legitimacy or constitutionality of a law. That is up to the Supreme Court. Also, the President cannot pass a law on his or her own; it usually needs to go through the House and Senate
The most effective way for Presidents to overcome limitations to their power is for there to be an emergency nationwide. Then the President has an excuse to pass executive orders. Also, the House and Senate could work together to give the President more power.
For example, Abraham Lincoln suspended habeas corpus during the emergency of the Civil War. Typically Presidents do not do this and his excuse was that it was an emergency to combat the rebels.
