The official said members of the Intelligence Committees would now get “access” to the documents.

Christopher Anders, senior legislative counsel for the American Civil Liberties Union, called the president’s move “a small step in the right direction.” But he noted that the legal memo or memos were not being shared with the Armed Services Committees, which have jurisdiction over Pentagon strikes, or the Judiciary Committees, which oversee the Justice Department. It was not clear whether the release involved more than one memo.

The public should be permitted to see at least a redacted version of the relevant material, Mr. Anders said. “Everyone has a right to know when the government believes it can kill Americans and others,” he said.

The Senate Intelligence Committee is expected to closely question Mr. Brennan about his role in the drone program during his hearing. Senator Ron Wyden, an Oregon Democrat who sits on the committee, said in a phone interview that he had been working in his office on questions for Mr. Brennan about 6:30 p.m. Wednesday when Mr. Obama called him and said that “effective immediately he was going to make the legal opinions available and he also hoped that there could be a broader conversation.”

Mr. Wyden has repeatedly called on the administration to release its legal memorandums laying out what the executive branch believes it has the power to do in national security matters, including the targeted killing of a citizen. Earlier on Wednesday, at a Democratic retreat in Annapolis, Md., he had hinted at a potential filibuster of Mr. Brennan’s nomination by vowing to “pull out all the stops to get the actual legal analysis, because without it, in effect, the administration is, in effect, practicing secret law.”

Mr. Wyden said that committee members would have immediate access to the material, and that there would be a process for other senators to read it eventually. It was not clear whether lawmakers’ legal aides would also be allowed to read it.

He said the administration’s decision to allow lawmakers “to finally see the legal opinions” was an “encouraging first step, and what I want to see is a bipartisan effort to build on it, particularly right now, when the lines are blurring between intelligence agencies and the military.”