The US Supreme Court is set to make a landmark ruling on military trials for Guantanamo Bay detainees.
Hamdan was a driver for Osama Bin Laden
Osama Bin Laden's ex-driver, Salim Ahmed Hamdan, is arguing that US President George W Bush did not have the authority to order such trials.
He is demanding a civilian trial or court martial, where the prosecution has to work harder than at a tribunal to make its case.
Mr Hamdan is one of 10 inmates facing trial by military tribunal.
If the Supreme Court rules in his favour, most legal experts agree that cases against Guantanamo inmates could be very hard for the US government to win, given a lack of admissible evidence, says the BBC's Justin Webb in Washington.
The ruling could also determine the future of Guantanamo Bay camp itself, which has faced growing calls internationally to be closed down.
The Cuba-based facility currently holds about 460 inmates, mostly without charge, whom the US suspects of links to al-Qaeda or the Taleban.
Mr Hamdan had success in his first legal outing, in the US District Court in Washington, which ruled that he could not face a military trial unless he had previously been found not to be a prisoner of war under the Geneva Convention.
He claims POW status, but like all camp prisoners, he is denied this and is instead designated an "unlawful combatant" by the Bush administration.
However, an appeal court reversed this decision and said Mr Bush had the authority to order the trials.
The tribunals have been criticised by rights groups as unconstitutional.