The first to break ranks was P J Crowley, a state department spokesman, who last month denounced the Pentagon's treatment of Bradley Manning as "ridiculous and counterproductive and stupid". He then resigned. Now, more than 250 American scholars have attacked the conditions of Manning's imprisonment at the military brig in Quantico, Virginia, in a letter to the New York Review of Books, stating that they are in "violation of the Eighth Amendment's prohibition of cruel and unusual punishment and the Fifth Amendment's guarantee against punishment without trial".

If continued, the letter states, the treatment "may well amount to a violation of the criminal statute against torture, defined as, among other things, 'the administration or application . . . of . . . procedures calculated to disrupt profoundly the senses or the personality' ".

The principal signatories are Bruce Ackerman and Yochai Benkler from Yale and Harvard Law Schools, respectively. But further down the list there is a crucial name: Laurence H Tribe, a leading expert on constitutional law, former professor to Barack Obama and a supporter of his old student's campaign for the presidency. The direct attack on Obama is therefore all the more pointed:

President Obama was once a professor of constitutional law, and entered the national stage as an eloquent moral leader. The question now, however, is whether his conduct as commander-in-chief meets fundamental standards of decency. He should not merely assert that Manning's confinement is "appropriate and meet[s] our basic standards", as he did recently. He should require the Pentagon publicly to document the grounds for its extraordinary actions – and immediately end those that cannot withstand the light of day.

Last month I met David House, a friend of Manning and the only person apart from his lawyer who is able to visit him in prison. He described how, over the past few months, Manning's condition has rapidly declined. Held in solitary confinement for 23 hours a day under a Prevention of Injury order (which Brig psychiatrists, based on their psychological assessments of Manning, say there is no need for), Manning has become increasingly non-communicative, verging on the catatonic.

House finds it increasingly difficult to engage him in coherent conversation (the two men used to bond over a shared love of technology and politics). "I can't really describe how bizarre it is to see a 110-pound, five-foot-three individual done up in chains from his hands to his feet, connected at the waist, so he can't really move," House told me.

The hope must be, as the personal pressure builds on Obama, that the president recognises how inconsistent the degrading and inhumane treatment of Manning is with the values he apparently sought to instil in public life when he began his presidency.

Now that America's academic fraternity has launched itself into the debate and directly questioned Obama's morality, he can surely no longer hide behind Pentagon reassurances that Manning's treatment is "appropriate and meets our basic standards". Especially not when it is so obviously neither.