Attorney Neal Katyal, who represents Salim Ahmed Hamdan, told justices the military commissions established by the Pentagon on Mr. Bush's orders are flawed because they violate basic military justice protections.
"This is a military commission that is literally unburdened by the laws, Constitution and treaties of the United States," Katyal said.
Defense attorneys also argued that the one conspiracy charge lodged against Hamdan did not exist in law until the Pentagon invented it, CBS News correspondent Wyatt Andrews reports.
Justices Antonin Scalia and Samuel Alito, the newest member of the court, pressed Katyal to explain why a defendant before a military commission should be given something that defendants in civilian criminal trials normally don't get — the chance to challenge the case before a verdict is reached.
"If this were like a (civilian) criminal proceeding, we wouldn't be here," Katyal said.
At stake is more than whether Hamdan, after nearly four years at the Navy prison in Guantanamo Bay, goes on trial for war crimes before a special military commission, CBS News reports. Analysts say if the high court rejects Mr. Bush's plan to hold such trials, it could rein in the president's expanded powers in pursuing and punishing suspected terrorists.
But the administration argued that presidents since George Washington have used tribunals for battlefield justice, Andrews reports.
"The use of military commissions to try enemy combatants has been part and parcel of the war power for 200 years," Solicitor General Paul D. Clement said.
Scalia's presence on the bench signaled his rejection of a request to recuse himself that was filed Monday by five retired generals who support Hamdan's arguments. In a letter to the court, the generals asked Scalia to withdraw from participating in the case because of remarks he made in a recent speech in Switzerland about "enemy combatants." Speaking at the University of Freiberg in Switzerland on March 8, Scalia said foreigners waging war against the United States have no rights under the Constitution.
Still on the subject of Guantanamo Bay, Scalia also referred to the war in Iraq, saying: "I have a son on that battlefield and they were shooting at my son and I am not to give this, this man who is captured in a war, a full jury trial."
Alito also suggested the justices should wait until Hamdan's trial is over to allow him to question whether charging him with conspiracy violates the laws of war, as Katyal contends.
But Katyal brushed aside the contention. "The government has had four years to get their charges together on Mr. Hamdan," he said.