In a 42-page legal analysis, the Justice Department cited the Constitution, the Federalist Papers, the writings of presidents both Republican and Democratic, and dozens of scholarly papers and court cases in justifying President Bush's power to order the N.S.A. surveillance program.

With the legality of the program under public attack since its disclosure last month, officials said Attorney General Alberto R. Gonzales ordered up the analysis partly in response to what administration lawyers felt were unfair conclusions in a Jan. 6 report by the nonpartisan Congressional Research Service. The Congressional report challenged virtually all the main legal justifications the administration had cited for the program.

Vice President Dick Cheney, meanwhile, once again defended the N.S.A. eavesdropping operation in a speech Thursday as "critical to the national security of the United States," even as House Democrats prepared to hold an unofficial hearing on Friday into a program that they charge is illegal and unconstitutional. Mr. Cheney is also scheduled to meet with Congressional leaders on Friday at a separate, closed-door briefing on the program.

When the Senate Judiciary Committee conducts an open hearing on the eavesdropping on Feb. 6, Attorney General Gonzales is expected to testify. The session organized for Friday by Democrats is intended to spotlight critics of the program; administration officials will not use that forum to offer a defense. The White House has invited some members of the House and Senate Intelligence Committees to attend a briefing on Friday, according to Rep. Jane Harman of California, the ranking Democrat on the House Intelligence Committee.

The analysis released Thursday by the Justice Department, with comments from lawyers throughout the department, expanded on the legal arguments made in two still-classified legal opinions as well as in a slimmer letter that the department sent to Congress last month.

The basic thrust of the legal justification was the same - that the president has inherent authority as commander in chief to order wiretaps without warrants and that the N.S.A. operation does not violate either a 1978 law governing intelligence wiretaps or the Fourth Amendment ban on unreasonable searches.

This month's Congressional Research Service report was particularly critical of the administration's claim that the N.S.A. program was justified by a resolution passed by Congress three days after the Sept. 11, 2001, attacks, authorizing the use of "all necessary and appropriate force" against those responsible for the terrorist acts.

The research service report found there was no indication that Congress intended to authorize warrantless wiretaps when it gave President Bush the authority to fight Al Qaeda and invade Afghanistan. But the Justice Department did not back away from its position in Thursday's report, saying the type of "signals intelligence" used in the N.S.A. operation clearly falls under the Congressional use-of-force authorization.

"The president has made clear that he will exercise all authority available to him, consistent with the Constitution, to protect the people of the United States," the report said.

The Congressional authorization on the use of force, it added, "places the president at the zenith of his powers in authorizing the N.S.A. activities."

But many critics of the program, which allows the agency to eavesdrop on international phone calls and e-mail messages to and from American citizens and others within the United States, said that they remained unconvinced.

"The administration's latest justification for circumventing the law to spy on Americans falls far short of answering the many questions Congress and the American people have about this activity," said Senator Harry Reid of Nevada, the Democratic leader. "That is why there have been bipartisan calls for administration officials to come to Congress to answer these questions and ensure that the Judiciary and Intelligence Committees can thoroughly investigate the administration's actions."

Attorney General Gonzales sent Thursday's document to Mr. Reid and to Senator Bill Frist of Tennessee, the majority leader. While the report did not go into many operational details of the program, it sought to bolster the case for the president to retain inherent power to order warrantless searches in the United States as part of the seeking of information on foreign agents.

That authority, the Justice Department analysis said, is consistent with a three-part test established by the Supreme Court in a 1952 case, Youngstown Sheet and Tube Company v. Sawyer, which struck down President Harry S. Truman's authority to seize the nation's steel mills in the name of national security.

Nor does the N.S.A. program conflict, the Justice Department said, with what many legal analysts had regarded as the exclusive authority for intelligence wiretaps under the Foreign Intelligence Surveillance Act, passed by Congress in 1978 in response to Watergate-era political abuses. Some presidential powers, particularly in the area of national security, are simply "beyond Congress' ability to regulate," it said.

Vice President Cheney, who was actively involved in the creation of the N.S.A. program and has been a vigorous advocate for expanded presidential power, echoed that in a speech on Thursday before the Manhattan Institute for Policy Research in New York.

While some current and former officials have challenged the value of the N.S.A. program in deterring an attack on American soil, the vice president said: "The activities conducted under this authorization have helped us to detect and prevent possible terrorist attacks against the American people. As such, this program is critical to the national security of the United States."

President Bush and Mr. Cheney have been critical of the public disclosure of the program in The New York Times, and the Justice Department has opened an investigation into the disclosure. Mr. Cheney acknowledged in his speech that "a spirited debate is now under way, and our message to the American people is clear and straightforward: These actions are within the president's authority and responsibility under the Constitution and laws, and these actions are vital to our security."

But Robert Reinstein, dean of the law school at Temple University, said in an interview that he considered the eavesdropping program "a pretty straightforward case where the president is acting illegally," and he said there appeared to be a broad consensus among legal scholars and national security experts that the administration's legal arguments were weak.

The foreign intelligence law passed by Congress in 1978 represents the Bush administration's biggest legal hurdle, he said. "When Congress speaks on questions that are domestic in nature, I really can't think of a situation where the president has successfully asserted a constitutional power to supersede that," he said.

Two leading civil rights groups brought lawsuits this week aimed at ending the N.S.A. program, and several lawyers representing defendants in terrorism cases are also seeking to challenge the program on the grounds that it may have been improperly used in criminal prosecutions.

Mr. Reinstein predicted that the court would ultimately declare the program unconstitutional. "This is domestic surveillance over American citizens for whom there is no evidence or proof that they are involved in any illegal activity, and it is in contravention of a statute of Congress specifically designed to prevent this," he said.

Attorney General Alberto Gonzales, who plans to testify at a Senate Judiciary Committee hearing on February 6, sent a report to Capitol Hill outlining the legal basis for the National Security Agency's activities that Bush approved after the September 11 attacks.

The highly classified program allows the monitoring of international communications, like telephone and e-mail messages, into and out of the United States of persons linked to al Qaeda or related terrorist groups, without a warrant.

Disclosure last month of the program sparked an outcry by Democrats and Republicans, with many lawmakers questioning whether it violated the U.S. Constitution. Civil liberties groups filed lawsuits challenging the program's legality.

A 1978 law, the Foreign Intelligence Surveillance Act, makes it illegal to spy on U.S. citizens in the United States without the approval of a special, secret court. Bush secretly gave the NSA authority to intercept the communications without such approval.

"These NSA activities are lawful in all respects," Gonzalez said in a letter to Senate leaders in releasing the Justice Department's 42-page legal analysis.

"They represent a vital effort by the president to ensure that we have in place an early warning system to detect and prevent another catastrophic terrorist attack on America," he said.

Letter to Cheney

Four top congressional Democrats sent a letter to Vice President Dick Cheney on Thursday, asking the administration to expand beyond a small group of lawmakers briefings about the domestic eavesdropping program to ensure adequate oversight.

"We ask that all future consultations with Congress on this program be open to all members of the Senate and House intelligence committees," said the letter signed by Senate and House Democratic leaders, Nevada Sen. Harry Reid and Rep. Nancy Pelosi of California, and the senior Democrats on the Senate and House intelligence panels.

The American Civil Liberties Union, one of the groups challenging the program, rejected the administration's legal justifications.

"Any opinion coming from the Justice Department has to be viewed with a healthy dose of skepticism," said Anthony Romero, the group's executive director. "Congress must hold open, substantive hearings to let the American public know how their privacy was invaded."

Gonzales maintained that Bush's use of his authority to approve the eavesdropping program was "consistent" with the 1978 law.

He said the program "is also fully protective of the civil liberties guaranteed" by the Constitution protecting against unreasonable searches and seizes of evidence.

Besides Bush's constitutional power as commander in chief, Gonzales said the authorization of military force by the U.S. Congress after the September 11 attacks gave Bush the authority for the domestic surveillance.

A January 5 study by the Congressional Research Service, a nonpartisan research arm of Congress, questioned the administration's legal defense of the program but came to no firm conclusion about its legality because so many underlying facts are classified.

The study said it was unlikely Congress had expressly authorized Bush to conduct warrantless surveillance and suggested that Congress had intended the 1978 law to govern electronic surveillance during wartime.

"The history of Congress's active involvement in regulating electronic surveillance within the United States leaves little room for arguing that Congress has accepted by acquiescence the NSA operations here at issue," the study said.