Sen. Arlen Specter said Sunday he believes that President Bush violated a 1978 law specifically calling for a secret court to consider and approve such monitoring. The Pennsylvania Republican branded Gonzales' explanations to date as "strained and unrealistic."
The top Democrat on the Senate Judiciary Committee, Vermont Sen. Patrick Leahy, predicted that the committee would have to subpoena the administration to obtain internal documents that lay out the legal basis for the program. Justice Department officials have declined, citing in part the confidential nature of legal communications.
Specter said he would have his committee consider such a step if the attorney general does not go beyond his prior statements and prepared testimony that the spying is legal, necessary and narrowly defined to fight terrorists.
"This issue of the foreign intelligence surveillance court is really big, big, big because the president, the administration, could take this entire program and lay it on the line to that court," Specter told NBC's "Meet the Press."
The Foreign Intelligence Surveillance Act of 1978 established legal procedures for conducting intelligence-related searches and surveillance inside the United States.
Specter said the FISA court "has really an outstanding record of not leaking, and of being experts. And they would be pre-eminently well-qualified to evaluate this program and either say it's OK or it's not OK."
Leahy charged that Mr. Bush misled the public when he said during the presidential campaign in April 2004 that his administration was following the law by getting warrants for wiretapping.
"I think ultimately we're going to have to subpoena them," Leahy said on CBS' Face the Nation, expressing doubt that lawmakers would get the material otherwise.
Under the National Security Agency program put in place after the attacks of Sept. 11, 2001, the government has eavesdropped, without seeking warrants, on international phone calls and e-mails of people within the United States who are deemed to be a terrorism risk.
CBS News correspondent Randall Pinkston reports that the president's defenders say that congressional approval of the war in Iraq gave him the authority for domestic surveillance, and throughout criticism, Mr. Bush has maintained he not only has the authority, but the duty to eavesdrop.
In testimony prepared for Monday's hearing, Gonzales argues that Mr. Bush had authority under a 2001 congressional resolution authorizing force in the fight against terrorism and that heeding the 1978 law would be too cumbersome.
"The terrorist surveillance program operated by the NSA requires the maximum in speed and agility, since even a very short delay may make the difference between success and failure in preventing the next attack," Gonzales said in statements obtained by The Associated Press.
Specter was not so sure.
"I believe that contention is very strained and unrealistic," Specter said. If the FISA law was inadequate, he said, Bush should have asked Congress to change it rather than ignore it. "The authorization for the use of force doesn't say anything about electronic surveillance."