''Do I have the legal authority to do this? And the answer is, absolutely,'' Bush said Monday.
But plenty of people disagreed with his conclusion.
Angry that the program had been revealed, Bush said it was an effective tool in disrupting terrorists. He said it was a shameful act for someone to have leaked details to the media.
Attorney General Alberto Gonzales said it was ''probably the most classified program that exists in the United States government'' - involving electronic intercepts of telephone calls and e-mails in the United States of people with known ties to al-Qaida and other terrorist groups.
At a news conference, Bush bristled at the suggestion he was assuming unlimited powers.
''To say 'unchecked power' basically is ascribing some kind of dictatorial position to the president, which I strongly reject,'' he said angrily in a finger-pointing answer. ''I am doing what you expect me to do, and at the same time, safeguarding the civil liberties of the country.''
Despite Bush's defense, there was a growing storm of criticism from Congress and calls for investigations, from Democrats and Republicans alike. West Virginia Sen. Jay Rockefeller, the top Democrat on the Senate Intelligence Committee, released a handwritten letter expressing concern to Vice President Dick Cheney after being briefed more than two years ago.
Rockefeller complained then that the information was so restricted he could not fully evaluate, much less endorse these activities.'' He expressed concern about the administration's direction on security, technology and surveillance issues.
Sen. Arlen Specter, R-Pa., chairman of the Judiciary Committee, said he would ask Bush's Supreme Court nominee, Samuel Alito, his views of the president's authority for spying without a warrant.
Bush challenged Senate Minority Leader Harry Reid, D-Nev., and Sen. Hillary Rodham Clinton, D-N.Y. - without naming them - to allow a final vote on legislation renewing the anti-terror Patriot Act, saying it was inexcusable to let it expire. ''I want senators from New York or Los Angeles or Las Vegas to go home and explain why these cities are safer'' without the extension, he said.
Reid and Clinton both helped block passage of the legislation in the Senate last week.
On another issue, Bush acknowledged that a prewar failure of intelligence - claiming Saddam Hussein possessed weapons of mass destruction - has complicated the U.S. ability to confront other potential emerging threats such as Iran.
''Where it is going to be most difficult to make the case is in the public arena,'' Bush said. ''People will say, if we're trying to make the case on Iran, 'Well, if the intelligence failed in Iraq, therefore, how can we trust the intelligence on Iran?'