WASHINGTON – Senators raised doubts about the legal rationale for the Bush administration’s wiretapping program yesterday, forcing Attorney General Alberto Gonzales to provide a lengthy defense of the operations he called a vital “early warning system” for terrorists.

A handful of Republicans joined Democrats in raising questions about whether President Bush went too far in ordering the National Security Agency’s monitoring operations. The senators were particularly troubled by the administration’s argument that a September 2001 congressional resolution approving use of military force covered the surveillance of some domestic communications.

“The president does not have a blank check,” said Judiciary Chairman Arlen Specter, a Republican of Pennsylvania, who wants the administration to ask the secretive Foreign Intelligence Surveillance Court to review the program.

“You think you’re right, but there are a lot of people who think you’re wrong,” Mr. Specter told Gonzales. “What do you have to lose if you’re right?”

Mr. Gonzales didn’t respond to Mr. Specter’s proposal directly. “We are continually looking at ways that we can work with the FISA court in being more efficient and more effective,” said the former Texas judge.

Under Mr. Bush’s orders, the ultra-secret National Security Agency has been wiretapping – without warrants – on international communications of people in America whose calls and e-mails may be linked to Muslim extremists.

During the day-long committee hearing, Mr. Gonzales and the senators reached as far back as eavesdropping ordered by President Washington and delved into court decisions surrounding presidential powers and the 1978 Foreign Intelligence Surveillance Act.

Mr. Gonzales repeatedly defended the current program as lawful, reasonable, and essential to national security. He said the president’s authority was strongest in a time of war, and he called the monitoring operations an “early warning system designed for the 21st century.” He said no changes in law were needed to accommodate the monitoring.

“To end the program now would be to afford our enemy dangerous and potential deadly new room for operation within our own borders,” he said.

Democrats pressed Mr. Gonzales for details about the program and other similar operations, almost all of which he would not provide. They’ve asked Mr. Specter to file subpoenas for classified legal opinions on the subject.

“The president and the Justice Department have a constitutional duty to faithfully execute the laws,” the committee’s top Democrat, Senator Leahy of Vermont, said. “Nobody is above the law, not even the president of the United States.”

Mr. Leahy asked if the administration has authorized the opening of American citizens’ mail. Throughout the hearing, Mr. Gonzales chose his words carefully. “We’re only focused on international communications where one part of the communication is Al Qaeda,” he said.

Senator Feinstein, a Democrat of California, asked if the Bush administration had issued “any other secret order or directive” that would be prohibited by law. Said Mr. Gonzales: “The president has not authorized any conduct that I’m aware of that is in contravention of law.”

Republicans, too, were skeptical. Senator DeWine, a Republican of Ohio, said Mr. Bush’s power – and the country – would be stronger if he came to Congress for statutory authorization.

Senator Lindsey Graham, a Republican of South Carolina, said future presidents could be hurt when they seek authorizations to use force because the Bush administration interpreted Congress’ post 9/11-resolution so broadly.

And Senator Brownback, a Republican of Kansas, said he wanted to review whether changes were needed in the 1978 intelligence law to permit this type of monitoring.

Mr. Gonzales tried to paint eavesdropping as less heavy-handed than firing missiles or holding terrorists in detention. He noted the Supreme Court found it was appropriate to detain an American citizen for fighting alongside Al Qaeda. “How can it be that merely listening to Al Qaeda phone calls into and out of the country in order to disrupt their plots is not?” Mr. Gonzales asked.