The case involves Jose Padilla, who was arrested at Chicago's O'Hare International Airport in May 2002. The government says he plotted with al-Qaida leaders in Pakistan to detonate a radioactive bomb in an undetermined U.S. city.
But the goverment argues Padilla, who has not been charged with a crime, does not have the right to a lawyer or access to the U.S. courts after being declared an "enemy combatant" by the president.
"Never before in this nation's history has this court handed the president a blank check to imprison citizens," said Stanford law professor Jennifer Martinez, speaking on Padilla's behalf before the Supreme Court.
Deputy Solicitor General Paul Clement told the court a congressional resolution authorizing the president to use all necessary force after the Sept. 11 attacks also contained the authority to detain enemy combatants.