WASHINGTON, Dec. 19 - President Bush and two of his most senior aides argued Monday that the highly classified program to spy on suspected members of terrorist groups in the United States grew out of the president's constitutional authority and a 2001 Congressional resolution that authorized him to use all necessary force against those responsible for the Sept. 11 attacks.
Offering their most forceful and detailed defense of the program in a series of briefings, television interviews and a hastily called presidential news conference, administration officials argued that the existing Foreign Intelligence Surveillance Act was not written for an age of modern terrorism. In these times, Mr. Bush said, a "two-minute phone conversation between somebody linked to Al Qaeda here and an operative overseas could lead directly to the loss of thousands of lives."
Mr. Bush strongly hinted that the government was beginning a leak investigation into how the existence of the program was disclosed. It was first revealed in an article published on The New York Times Web site on Thursday night, though some information that administration officials argued could be useful to terrorists had been omitted.
"We're at war, and we must protect America's secrets," Mr. Bush said. "And so the Justice Department, I presume, will proceed forward with a full investigation."
He also lashed out again, as he did Saturday, at Democrats and Republicans in the Senate who have blocked the reauthorization of the broad antiterrorism law known as the USA Patriot Act, saying they voted for it after the Sept. 11 attacks "but now think it's no longer necessary."
Several of the senators responded that Mr. Bush would not accept amendments to the act that they say are necessary to protect civil liberties and that he would not accept a short-term renewal of the existing law while negotiations continue.
In the first of a series of appearances Monday to defend the intelligence operations, Attorney General Alberto R. Gonzales told reporters that "this electronic surveillance is within the law, has been authorized" by Congress. "That is our position," he added.
Officials with knowledge of the program have said the Justice Department did two sets of classified legal reviews of the program and its legal rationale. Mr. Gonzales declined to release those opinions Monday.
Two of the key Democrats who had been briefed on the program said Monday that they had been told so little that there was no effective Congressional oversight for it.
In a highly unusual move, Senator John D. Rockefeller IV of West Virginia released a letter he sent to Vice President Dick Cheney on July 17, 2003, complaining that "given the security restrictions associated with this information, and my inability to consult staff or counsel on my own, I feel unable to fully evaluate, much less endorse these activities." The letter was handwritten because secrecy rules prevented him from giving it to anyone to type.
On Monday Mr. Rockefeller said that after he sent his letter to Mr. Cheney, "these concerns were never addressed, and I was prohibited from sharing my views with my colleagues."
Senator Arlen Specter, the Pennsylvania Republican who is chairman of the Judiciary Committee, said, "I am skeptical of the attorney general's citation of authority, but I am prepared to listen."
Mr. Specter, who has said he will hold hearings on the program soon after the confirmation hearings for the Supreme Court nominee, Judge Samuel A. Alito Jr., said he did not believe the president's decision to inform a handful of members of Congress was sufficient.
"I think it does not constitute a check and balance," he said. "You can't have the administration and a select number of members alter the law. It can't be done."
Mr. Specter also predicted that the domestic spying debate would spill over into Judge Alito's confirmation. On Monday, he sent the judge a letter saying he intended to ask "what jurisprudential approach" the judge would use in determining if the president had authority to establish the program.
"The fat's in the fire," Mr. Specter said. "This is going to be a big, big issue. There's a lot of indignation across the country, from what I see."
Mr. Bush, Mr. Gonzales and Lt. Gen. Gen. Michael V. Hayden, the nation's second-ranking intelligence official and a former director of the National Security Agency, which conducted the surveillance, stepped around questions about why officials decided not to use emergency powers they have under the existing foreign surveillance law. The law allows them to tap international communications of people in the United States and then go to a secret court up to 72 hours later for retroactive permission.
"The whole key here is agility," General Hayden said, adding that the aim "is to detect and prevent."
Administration officials, speaking anonymously because of the sensitivity of the information, suggested that the speed with which the operation identified "hot numbers" - the telephone numbers of suspects - and then hooked into their conversations lay behind the need to operate outside the old law.
Soon after Mr. Bush spoke, three senior Democrats influential on national security matters - Senators Carl Levin of Michigan, Jack Reed of Rhode Island and Russell Feingold of Wisconsin - assailed the president for bypassing the court that Congress set up a quarter-century ago to make sure intelligence agencies do not infringe on the privacy of Americans.
"He can go to the court retroactively," Mr. Levin, the ranking Democrat on the Armed Services Committee, told reporters, referring to the 72-hour rule.
Mr. Bush - who initially resisted a public investigation into the Sept. 11 attacks and the intelligence failures in Iraq - used his news conference Monday to discourage Congress from publicly delving into the program, saying that "public hearings on programs will say to the enemy, 'Here's what they do, adjust.' " He repeatedly cited the case of Osama bin Laden, who was widely reported to have stopped using a satellite telephone after news reports that intelligence agencies were listening in.
The White House briefing itself was unusual, with two of the administration's most senior officials discussing legal and operational details of what Mr. Gonzales described as "probably the most classified program that exists in the United States government."
Mr. Gonzales said the president had "the inherent authority under the Constitution" as commander in chief to authorize the program. He also argued that the legal rationale followed the logic in a Supreme Court decision last year in the case of an enemy combatant named Yaser Esam Hamdi, an American citizen who was detained in Afghanistan on the battlefield.
In addition, Mr. Gonzales said the administration believed that Congress gave the president clear and broad authorization to attack Al Qaeda in a resolution passed on Sept. 14, 2001, that set the stage for the invasion of Afghanistan. That resolution authorized the president "to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on Sept. 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons."
Many members of Congress say that in authorizing the military invasion of Afghanistan days after the Sept. 11 attacks, they never intended or envisioned that the authority could be applied to searches without warrants within the United States.
Mr. Gonzales and General Hayden were careful to emphasize that the surveillance program was "limited" in scope.
"People are running around saying that the United States is somehow spying on American citizens calling their neighbors," Mr. Gonzales said. In fact, he said, it was "very, very important to understand" that the program is limited to calls and communications between the United States and foreign countries.
"What we're trying to do is learn of communications, back and forth, from within the United States to overseas members of Al Qaeda," he said. "And that's what this program is about."
He added: "This is not about wiretapping everybody. This is about a very concentrated, very limited program focused on gaining information about our enemy."
As the administration has argued since the disclosure of the program Thursday night, Mr. Gonzales and General Hayden said the normal system for issuing warrants for a domestic surveillance operation - required in 1978 in a program that grew out of the improper surveillance of political dissidents - was inadequate in some cases.
Regarding a possible leak investigation, which would be handled by the Justice Department, Mr. Gonzales said: "This is really hurting national security, this has really hurt our country, and we are concerned that a very valuable tool has been compromised. As to whether or not there will be a leak investigation, we'll just have to wait and see."
When questions at the news conference turned to Iraq, Mr. Bush urged reporters to look at rationales he offered for invading the country that went beyond its suspected caches of weapons of mass destruction, including his vision of creating democratic havens in the Middle East. But he acknowledged that the failure to find weapons in Iraq made it difficult to make the case "in the public arena" that countries like Iran are pursuing nuclear weapons, as Mr. Bush has charged.
He said, "People will say, if we're trying to make the case on Iran, well, the intelligence failed in Iraq, therefore how can we trust the intelligence in Iran?" Later, he added, "It's no question that the credibility of intelligence is necessary for good diplomacy."