The powers are granted by the Constitution.
There are 286,000,000 American citizens and three of them were scumbags who betrayed their country. In George Washington's day, they would have been hung.
Hamdi is an enemy combatant, his citizenship is moot. He chose to be an enemy combatant. Screw him.
If a future President begins jailing his politcal foes because of their ideology then that becomes a political question and if it gets bad enough then you lock and load.
Since when did the presence (or absence) of a law keep the Clintons from skirting the law or abusing their authority? They did what they wanted, whether the law was on their side or not.
To lawbreakers, laws are no obstacle. Only the lawful are constrained by the law.
The best way, the only way, to protect the country from such abuse is to elect political leaders of character -- people who can be trusted with the power of office.
Ergo, no more Clintons!