The high court is hearing arguments in two related cases: Yaser Hamdi, captured on the battlefield in Afghanistan, and Jose Padilla, arrested in Chicago on suspicion of plotting a terrorist act.
In both cases, the court is being asked whether constitutional protections against being locked up without trial apply in the war on terrorism.
Hamdi and Padilla have been locked up in solitary—with no charges filed— for most of the last two years. Their lawyers told the Supreme Court that even in war, you can't do that to American citizens, period, reports CBS News Correspondent Wyatt Andrews.
"We have never authorized the detention of a citizen in this country without giving him an opportunity to be heard. To say, 'Hey, I am an innocent person,'" Hamdi's attorney Frank Dunham said.
"We could have people locked up all over the country tomorrow, with no opportunity to be heard. … Congress didn't intend for widespread, indefinite detentions," Dunham told Justice Sandra O'Connor.
O'Connor wondered whether the president was granted detention power when Congress approved the use of military force shortly after the Sept. 11, 2001 attacks.
"Nowhere does the (statute) have 'detention' in it," Dunham said.
Most of the justices revealed a strong respect for the president's power to detain an enemy combatant even if the combatant is an American, but reports Phillips, the constitutional questions now are, can this power ever be challenged and is it forever?
But Bush administration attorney Paul Clement argued that a president as commander in chief has wide latitude under constitutional law to detain suspected terrorists as "enemy combatants" if they pose a national security risk.
"It has been well-established, and long established, that the government has the authority to hold unlawful enemy combatants … in order to prevent them from returning to the field of battle," he said.
Hamdi was born in Louisiana while his Saudi father worked there, but grew up in the Middle East. Padilla is a convert to Islam who was born and raised in Chicago and spent time in prison.
Both are U.S. citizens.
The Bush administration says they also are "enemy combatants," dangerous enough to warrant open-ended military detention, perhaps for the duration of the open-ended war on terror.
The line for scarce seats in the courtroom to hear arguments began forming early Tuesday evening. Would-be spectators camped out in 40-degree weather, huddled in blankets and parkas.
Brian Swenson, 24, of Washington, said he came out to see "whether the Constitution can be manipulated."
"I'm sympathetic to Padilla," he said. "If you're locked up you ought to be given a trial."
The government is holding Hamdi and Padilla in near isolation at a Navy brig in South Carolina. Until recently neither had seen his lawyer or known that his case was before the Supreme Court.
Hamdi was captured weeks after the Sept. 11 attacks. The government says he was fighting with Taliban forces in Afghanistan, home base of the al Qaeda terrorist network, and calls him a classic battlefield detainee.
He initially was housed with hundreds of alleged foreign fighters at a Navy base in Guantanamo Bay, Cuba, but transferred to the United States when authorities verified his citizenship claim.
Padilla was arrested two years ago in Chicago on suspicion of plotting to detonate a radioactive bomb. He was first placed in custody of civilian authorities, but was transferred to military detention in June 2002.
In legal filings, the Bush administration said it has unilateral authority to order enemy combatant seizures and detentions, even inside the United States, but added that Congress also signed off on that course. Congress approved use of military force a week after the terrorist attacks of Sept. 11, 2001.
"Congress was acting in immediate response to attacks carried out within the nation's borders" when it voted to endorse broad White House authority to go after terrorists, administration lawyers argued in a court filing.
Lawyers for Padilla say Congress did no such thing and the White House has overstepped its authority.
"If the government's position were accepted, it would mean that for the foreseeable future, any citizen, anywhere, at any time, would be subject to indefinite military detention on the unilateral order of the president," their filing said.
The Bush administration won its arguments in lower courts in the Hamdi case, but lost a federal appeals court fight in the Padilla case.
The Hamdi and Padilla cases echo World War II-era cases about prisoners of war and war criminals, but also take the Supreme Court into uncharted waters. No one knows when the war on terror will end, and it is unclear if the justices will view it as a war at all.
These cases will determine how much deference the judiciary intends to give to the executive branch in this time of war and terror,". "They will determine, in effect, how much we must sacrifice individual liberties in the name of collective security; how far the protections of the Bill of Rights will be required to bow to the exigencies of the current crisis."
The cases are the last the court will hear this term. Another arising from the government response to the Sept. 11 attacks was heard last week. Lawyers for the foreign-born suspectsargued they should be give access to U.S. courts.