Roberts said after the open session resumed, "We have agreed to do what we already agreed to do, and that is to complete as best we can phase two of the Intelligence Committee's review of prewar intelligence in reference to Iraq."
Republicans complained that going into closed session interrupted important business.
But Democrats said they were prepared to invoke Rule 21 daily to put pressure on the GOP leadership for the intelligence investigation and other oversight matters.
Durbin said it was the first time that Rule 21 had been invoked in more than 25 years. Although closed sessions have been held from time to time more recently -- the Senate shut its doors last year to discuss intelligence-gathering -- the previous closures were done by agreement of both parties.
"The purpose of this closed session is to discuss the need for a phase two investigation by the Senate Intelligence Committee," Durbin told reporters after the move. He said this investigation was promised as early as Feb. 12, 2004, yet "nothing has been done."
"It is clear now that the American people were not informed properly before the invasion of Iraq," he said. "Intelligence information was distorted, was misused, and we have seen as late as last week the lengths which this administration has gone to try to silence and discredit their critics of the misuse of this intelligence information."
Durbin added, "We're serving notice on [Senate Republicans] at this moment: Be prepared for this motion every day until you face the reality. The Senate Intelligence Committee has a responsibility to hold this administration accountable for the misuse of intelligence information. They have promised this investigation. We will continue to make this request until they do it."
Under Rule 21, the chamber can be ordered into closed session when any member calls for it and the motion is seconded. No vote is required. Going back into regular session requires a majority vote.
Sen. Charles E. Schumer (D-N.Y.) called the move "historic" and said it was prompted by mounting frustration.
"An administration that can't admit it makes mistakes is bound to make many, many, many more of them," he said. The closed session was needed to "get the majority to implement its proper oversight role," he said. "That relates to the war, and it relates to many, many others issues as well."