WASHINGTON, Feb. 6 — President Bush decided against allowing the National Security Agency to intercept purely domestic phone calls and e-mail messages after the Sept. 11 attacks in part because officials realized such a decision would provoke intense opposition if made public, Attorney General Alberto R. Gonzales testified Monday.
Mr. Bush was intent on striking "the right balance" between national security and privacy interests, Mr. Gonzales said at the start of Congressional hearings into the N.S.A. program.
Noting the criticism the program has drawn since its disclosure in December, Mr. Gonzales said, "the reaction would have been twice as great" if Mr. Bush had expanded it to eavesdrop on communications wholly inside the country.
Under the surveillance program, the N.S.A. has been eavesdropping without warrants on the telephone calls and e-mail messages between people inside the United States and people overseas.
Mr. Gonzales's comments offered a new window into the political and legal calculus at the White House involving the decision to authorize the surveillance without court approval.
Echoing arguments the administration has made repeatedly in recent weeks, he said the president had the power to order the eavesdropping under both the Constitution and a Congressional resolution authorizing the use of force to fight terrorism.
His testimony came against a backdrop of intense partisan wrangling, with Democrats calling the program illegal and accusing the administration of misleading them about it in an effort to keep it secret. Some Republicans on the panel also expressed skepticism about the program's legal underpinnings, but most defended the president's authority to pursue terrorists aggressively and prevent another attack.
Mr. Gonzales sidestepped numerous questions about how the program operated. He would not say, for instance, how many American citizens have been the targets of the eavesdropping operation, or exactly when the program started. His refusal to discuss details exasperated Democrats, who were also frustrated that he refused to answer other queries that he termed "hypothetical" about any limits to the president's powers.
He made it clear that he was uncomfortable discussing the N.S.A. program at all because he said the public inquiry could tip off Al Qaeda to tactics in use by American intelligence agents.
"Our enemy is listening," he said, "and I cannot help but wonder if they aren't shaking their heads in amazement at the thought that anyone would imperil such a sensitive program by leaking its existence in the first place, and smiling at the prospect that we might now disclose even more or perhaps even unilaterally disarm ourselves of a key tool in the war on terror."
The contentious tone for the debate Monday was clear in several sharp exchanges, including one in which Sen. Patrick J. Leahy, Democrat of Vermont, frustrated at not getting answers, said, "Of course, I'm sorry, Mr. Attorney General, I forgot you can't answer any questions that might be relevant to this."
But while Democrats led the attack on the surveillance program, several Republican senators — including Mike DeWine, Lindsey Graham, Sam Brownback and Arlen Specter — also raised concerns. On the program's legality, Mr. Specter told the attorney general, "You think you're right, but there are a lot of people who think you're wrong."
Several Republicans also questioned why the administration could not have worked within the Foreign Intelligence Surveillance Act, or FISA, created by Congress in 1978 after Watergate to authorize warrants for eavesdropping, or did not seek expanded and explicit authority from Congress to conduct eavesdropping without warrants.
Senator Graham and others also questioned the Bush Administration's assertions that the Congressional resolution authorizing the use of force in the fight against terror, passed in the days after the Sept. 11 attacks, provides the legal basis for the N.S.A. program.
"The short answer is, we didn't think we had to, quite frankly, " Mr. Gonzales said.
Mr. Gonzales said that while the FISA system was at times too cumbersome to allow for the fast detection of terrorist threats, the administration still considered it a valuable tool and was using it aggressively to conduct purely domestic eavesdropping.
Indeed, he disclosed that the number of warrants issued by the foreign intelligence court rose 18 percent in 2005 from 2004, when more than 1,700 were issued.
Much of the concern from Democrats focused on what Senator Dianne Feinstein of California called the "slippery slope" created by the N.S.A. program, which they said could allow a president to claim almost limitless power to fight terrorism, with no checks from Congress.
Mr. Gonzales said that as commander in chief, the president's powers "are not limitless. Obviously Congress has a role to play in time of war."
But when asked to elaborate on where those limits lay, he refused to be drawn into specifics. He was asked several times, for instance, whether the president had or could authorize without warrants the interception of communications entirely within the United States, and he was careful to say that such interceptions were "beyond the bounds of the program which I'm testifying about today."
He acknowledged that the administration considered whether to allow eavesdropping without warrants on domestic to domestic communications but decided against it. That restriction drew skepticism from some lawmakers, who wanted to know why a program that the White House considered so vital to national security would not be used to track the communications of people within the United States. Mr. Gonzales himself acknowledged the apparent inconsistency.
"I suspect there are some in America who are saying, 'Well, why aren't you? If you've got reason to believe that you've got two members of Al Qaeda talking to each other in America, by God, why aren't you listening to their conversations?' " the attorney general said. "Again, this was a judgment made that this was the right balance between the security of our country and protecting the private interests of Americans."
Mr. Gonzales said that the president's authorization for the N.S.A. program came within weeks of the Sept. 11 attacks, which was earlier than previously known. He refused to give a specific date for the authorization but said that it came after Sept. 14, 2001, when Congress approved the use of force against those responsible for the attacks, and before the antiterrorism law known as the USA Patriot Act was approved some five weeks later. He said he did not believe the administration went to Congress as part of the Patriot Act debate to seek expanded authority to conduct wiretaps without warrants, an acknowledgment that drew criticism from Senator Leahy. The administration went to leaders of the Intelligence Committees in 2004, but met resistance about changing FISA, Mr. Gonzales said.
After his daylong testimony, Mr. Gonzales still faced skepticism from Republicans, notably Mr. Specter, the committee chairman. He said he planned to hold a second day of testimony with Mr. Gonzales.
At the end of the hearing, Mr. Specter pronounced a harsh verdict: he was not persuaded by the administration's case.
Mr. Specter said the attorney general's interpretation of the FISA statute "just defies logic and plain English." Similarly, he said that no "fair reading" of the September 2001 Congressional resolution authorizing military force could justify electronic surveillance.
Noting that Mr. Gonzales had said he could not discuss any operational details of the N.S.A. program, he expressed the hope that a closed hearing of the Senate Intelligence Committee set for Thursday would "come down to brass tacks."
The presiding judge of the FISA court was told in secret about the program in 2002, and other judges on the court have been briefed since its public disclosure. Mr. Specter urged that the administration let the court "pass judgment" on the legality of the program.
"Because if they disagree with you," he said, "it's the equilibrium of our constitutional system which is involved."