Legal and security experts continue to debate administration arguments that President Bush had the authority to tell the National Security Agency to eavesdrop on international calls starting or ending on American soil without a court warrant.

"There isn't any question in my mind that this operation violated federal law. And once you determine that federal law was violated, you ... reach a very troubling set of related conclusions," George Washington University law professor Jonathan Turley told FOXNews.com on Tuesday.

"I think there's a very powerful case that the president has independent authority" to order surveillance without a warrant, said Robert Turner, a University of Virginia professor who specializes in national security law.

President Bush on Saturday acknowledged the existence of the secret program that gave the NSA permission to listen in on hundreds, maybe thousands, of phone and e-mail conversations involving people believed to be members or associates of Al Qaeda. The president has said the program is necessary to protect national security, and he and other administration officials say they stand on firm legal ground in the authorization of the program.

"It is good, solid, sound policy. It is, I'm convinced, one of the reasons we have not been attacked for the last four years. It's absolutely the right thing to do," Vice President Dick Cheney said Tuesday while traveling in Pakistan.

Cheney repeated administration arguments that the authority is "absolutely consistent with the Constitution," is reviewed every 45 days, has been approved by the Justice Department and is known among top congressional members with security clearances.

He added that if the NSA had the authority prior to Sept. 11, 2001, to listen in on terror suspects or affiliates who may be inside the United States then the agency may have been able to pick up on conversations between the hijackers and their overseas connections before the terror attacks.

Added Homeland Security Secretary Michael Chertoff: "In this war we don't have radar, we have intelligence." If one considers the "total set of tools that we have, whether it is the NSA monitoring programs, whether it is the Patriot Act, these tools are critical tools in defending this country against terror," Chertoff told FOX News.

But alarming many critics is the president's decision to authorize surveillance without what is widely seen as a basic step to preserve constitutional rights: obtaining a warrant through the approval of a judge. In 1972, the Supreme Court punted on the question of whether the president is allowed to authorize wiretaps of foreign powers or their agents without a court order, Turner told FOXNews.com.

Administration Cites Law, Court Precedent

President Bush and Attorney General Alberto Gonzales on Monday cited three areas in which the administration has the authority to conduct warrantless domestic surveillance: presidential powers in Section 2 of the U.S. Constitution; the 2001 congressional authorization for the use of force after the Sept. 11 attacks; and the Supreme Court's decision in the 2004 case of enemy combatant Yaser Hamdi, a Saudi-American citizen captured on the battlefield in Afghanistan who was held for three years without being charged.

On Tuesday, White House spokesman Scott McClellan repeated the legal underpinnings used to justify the "signals intelligence."

"Under Article 2 of the Constitution, as commander in chief, the president has that authority. The president has the authority under the congressional authorization that was passed and clearly stated that, quote, 'The president is authorized to use all necessary and appropriate force.' This was under Section 2 in the authorization for the United States Armed Forces," McClellan said.

"It is limited to people who have — one of the parties to the communication [who has] a clear connection to Al Qaeda or terrorist organizations and one of the parties [who] is operating outside of the United States. And I think that's important for people to know, because there's been some suggestions that it's spying inside the U.S. That's not the case," the press secretary added.

Gonzales told reporters that the Supreme Court decision on Hamdi reinforced the claim that the president was given wide permission in the Sept. 14, 2001, vote by Congress authorizing the president to "use all necessary and appropriate force" against those behind the Sept. 11 attacks.

Gonzales said the congressional authorization did not specifically mention the word "detention," but in the Hamdi case, Justice Sandra Day O'Connor wrote in the majority opinion "that detention of enemy soldiers captured on the battlefield ... had been authorized by the Congress when they used the words, 'authorize the president to use all necessary and appropriate force.'"

"We believe the court would apply the same reasoning to recognize the authorization by Congress to engage in this kind of electronic surveillance," Gonzales said.

The New York Times, which first disclosed the existence of the NSA program last week, also cited unnamed sources who said the administration used two other opinions to justify its actions. One was embedded in a public Justice Department brief from 2002 and another was in a 2002 opinion issued by the Federal Intelligence Surveillance Court of Review that oversees the secretive court that usually deals with terror-related wiretap requests.

In 2002, that FISA review court upheld the president's warrantless search powers, referencing a 1980 Fourth Circuit Court of Appeals decision. That court held that "the president did have inherent authority to conduct warrantless searches to obtain foreign intelligence information. ... We take for granted that the president does have that authority and, assuming that is so, FISA could not encroach on the president’s constitutional power," wrote the court.

"The Foreign Intelligence Court of Review, which is the highest court that's looked at these questions, has said that the president has the inherent constitutional authority to use electronic surveillance to collect foreign intelligence and Congress cannot take away that constitutional authority. That's a pretty good argument," Bryan Cunningham, former National Security Council legal adviser, told FOX News.

Cunningham offered several other circumstances under which FISA warrants would be unnecessary.

"If the physical interceptions were done outside the United States and if it were the communications of the foreign person that were targeted, not the person inside the United States, or if the person inside the United States was not found to be a U.S. person — that is a citizen or resident or permanent resident alien — then those circumstances would potentially take this out of FISA, and therefore, not require a FISA warrant," he said. "It principally depends on where the collection is being done."

In speaking to reporters Monday, Bush said he wanted to stress that the calls are not intercepted within the United States, but involve one party outside the country.

But Turley said none of the reasons the administration has provided disprove charges that it broke the law by failing to seek a warrant. He said he believes the president violated the Fourth Amendment, which prohibits unreasonable searches and seizures, as well as the separation of powers between the legislative and the executive branches.

"[Bush] is saying that his authority as president trumps that of Congress," Turley said. "I don't see any merit whatsoever to the president's arguments.

"Any time a president orders a clearly criminal act, it should automatically trigger questions of his fitness of office," he added.

Turley added that the Hamdi case fails to give the administration any authority because, while the court did uphold the administration's detention of Hamdi, the majority opinion also said Hamdi must be given the ability to go before a court. Turley said it's no different in granting permission for surveillance: judicial oversight is required.

However, Turner, who is assistant director for the Center of National Security Law at the University of Virginia, argued that history is on the president's side, especially before the Foreign Intelligence Surveillance Act of 1978 created a special, secret court to review surveillance requests.

"To the extent that the first 200 years of our history — basically 90 percent of our history when FISA didn't exist — everybody seemed to understand that this was exclusive presidential territory ... that can't be taken away by Congress by statute."

Turner said 1973's United States v. Brown and 1978's United States v. Humphrey — both appellate decisions — upheld the president's authority to perform warrantless wiretaps in foreign intelligence, but a 1972 case that reached the Supreme Court sidestepped the issue of domestic surveillance related to "foreign powers or their agents.”

"I can't say the Supreme Court would uphold what they have done," Turner said of the administration's decision. But "I don’t think the president clearly broke the law."

Turner said he thinks the president was doing what he needed to do to protect the public.

"This is a president trying desperately ... to find out what our enemies are planning and stop them," he said.

Demands for Legislative Oversight

Bush administration officials have acknowledged the NSA program, but have been sparse in offering details about it, including how many people have been targeted and the methods used to target them.

That hasn't stopped members of Congress in both parties who want administration officials to come before them and testify about the program. Sen. Arlen Specter, R-Pa., said he intends to hold hearings on whether the president violated federal law, and Sen. Barbara Boxer, D-Calif., said she's asking presidential scholars to see if Bush committed an impeachable offense.

"It is a ridiculous idea that this president would be impeached for executing his responsibility to protect America from terrorist attacks," said Sen. John Cornyn, R-Texas.

Sen. Lindsey Graham, R-S.C., a former Judge Advocate General attorney, said he thinks the president is doing his job by ordering wiretaps, but is concerned about presidential misuse of the law.

"Here's the danger, the next president who is asking for Congress to give him authority to use force, if you overly interpret that to say 'I can do anything that I want to do with other laws,' it's going to be harder to get Congress to give the president the force authorization," he said.

But Graham said he certainly doesn't want the events to become political when they need not be.

"There's going to be a building outrage in more of the liberal media than anywhere else about this, and there are going to be Democratic members of Congress who are going to deny any involvement here. If they start to do that, then it is important that we understand where politics begins and ends and where policy begins and ends," he told FOX News Radio.

Graham said he's bothered by the fact that Gonzales apparently came to Congress and asked for an amendment to FISA and was advised it wasn't possible without jeopardizing the program.

"It's really a big problem and I'd like to know how that happened. If we have fell down on our job shame on us because I had rather amend the act for the future of our country then to have executives orders setting aside the act as a routine policy," he said. "And I don't think it's fair to be briefed, to be asked to help and you stiff-arm the White House and then turn right around and blame them."

Cornyn added that in May 2004, FBI Director Robert Mueller testified before the Senate Judiciary Committee that the FISA process had become too slow.

"Speed is important when particularly you're talking about getting some surveillance of a cell phone conversation between an Al Qaeda operative here in the United States and their counterpart overseas," Cornyn said. "That's apparently the reason why this alternative procedure was used in a limited number of cases involving the agents of foreign powers, particularly Al Qaeda operatives."

FOX News' Jim Angle and Catherine Herridge contributed to this report.