The 50-46 vote on Wednesday means that Attorney General John Ashcroft will not be required to provide details of interrogation methods and enemy combatant treatment.
Democrats accused Republicans of cooperating with a White House cover-up of policies they said may have contributed to the scandals such as the one at Baghdad's Abu Ghraib prison.
Republicans had sought to kill the measure on a procedural vote, but five of their own senators sided with Democrats to defeat that effort and force a direct vote on the amendment.
Pressure to explain
Under pressure from lawmakers, the White House on Tuesday released newly declassified papers to try to demonstrate that George Bush and his top aides insisted that detainees at Guantanamo Bay be treated humanely.
"Let me make very clear the position of my government and our country: We do not condone torture. I have never ordered torture. I will never order torture," Bush said on Tuesday.
But Democrats said the documents were largely irrelevant and raised more questions than they answered.
Senator Richard Durbin, an Illinois Democrat, said material released so far showed "this administration engaged in a fierce and protracted debate about whether they could redefine torture ... and whether this president as commander in chief was above the law."