In remarks to reporters at the end of his visit to the heavily-secured NSA in suburban Maryland, Mr. Bush again argued that both the Constitution and acts of Congress give him the authority to order domestic eavesdropping of terrorist suspects without a warrant.
"We must learn the intentions of the enemies before they strike," Mr. Bush said. "That's what they do here. They work to protect us."
Mr. Bush cited last week's tape from Osama bin Laden, saying the terrorist threat remains real and the safety and security of the American people depend on the government's ability to find out who the terrorists are talking to and what they're saying.
He pledged to continue to reauthorize the program as long as a threat exists.
"We must be able to quickly detect when someone linked to al Qaeda is communicating with someone inside of America," Mr. Bush said, after speaking behind closed doors to NSA employees and going on a tour of the agency.
Democrats and other critics maintain that Mr. Bush already had that authority under the Foreign Intelligence Surveillance Act, passed by Congress in 1978, and that he could have proceeded with intelligence eavesdropping deemed emergency as long as he notified a FISA court within 72 hours to seek approval after the fact.
Mr. Bush's stop here had two purposes. He was aiming to boost the morale of the people carrying out the work of a four-year-old domestic spying program in which the government monitors the international communications of people inside the United States whom it believes to have connections to the terrorist network al Qaeda. The president is also leading a wide-ranging campaign by his administration to defend the program, under fire from Democrats and Republicans alike who argue that it may be illegal.
"We've seen that part of the terrorist strategy is to place operatives inside of our country. They blend in with the civilian population," Mr. Bush said. "They get their orders from overseas and then they emerge to strike from within.
"The American people expect me to protect their lives and their civil liberties."
The White House has said what it is doing is not a domestic spying program, since one end of the phone call or e-mail is always outside the United States. The administration argues that a more accurate term would be "terrorist surveillance program."
Sen. Hillary Rodham Clinton, D-N.Y., wasn't buying Mr. Bush's explanations.
"Obviously, I support tracking down terrorists. I think that's our obligation. But I think it can be done in a lawful way," she said. "Their argument that it's rooted in the authority to go after al Qaeda is far-fetched. Their argument that it's rooted in the Constitution inherently is kind of strange because we have FISA and FISA operated very effectively and it wasn't that hard to get their permission."
Hearings begin in less than two weeks on Capitol Hill about whether Mr. Bush has, as he says, the authority to allow the program.
Sen. John McCain said Wednesday morning that he's eager to learn more about the program.
"I'm not familiar with the scope of the program, but I think that the American people would strongly support the ability of the president to go after al Qaeda and use whatever technical means necessary in order to make sure we make America safe,"
"I think it's good that he has welcomed hearings," McCain said. "I don't know if it's legal or illegal, but I think the hearings will help clear that up."