Four leading Democratic senators sent Mr. Bush a letter Wednesday saying although they support efforts to do everything possible within the law to combat terrorism, the National Security Advisory program is an "apparent violation of federal law."
Also, the former head of the NSA when the Sept. 11 attacks occurred said had the president's domestic spying program been in place, some of the hijackers would have been "detected."
, Mr. Bush said employees there who are secretly monitoring phone calls and Internet traffic are learning what terrorists are plotting against America. Mr. Bush said they are taking Osama bin Laden seriously when he says he's going to attack again.
The visit was accompanied by a new White House line, casting the program as a vital military operation, one that cannot wait for courts to consider warrants, reports CBS News correspondent John Roberts.
"Do you expect our commanders in a time of war to go to a court while they're trying to survey -- surveil the enemy? I don't think so," White House press secretary Scott McClellan told reporters.
Referring to bin Laden, Mr. Bush said, "When he says he's going to hurt the American people again, or try to, he means it.
"I take it seriously, and the people of NSA take it seriously," he added.
It was Mr. Bush's first comment about bin Laden since a tape was aired last week in which the al Qaeda leader warned that his fighters are preparing new attacks in the United States.
Some experts and lawmakers from both parties have questioned whether it's legal for the government to listen to conversations in the United States without a warrant, which the administration could get through the Foreign Intelligence Surveillance Court.
"If you or officials in your administration believe that FISA, or any law, does not give you enough authority to combat terrorism, you should propose changes in the law to Congress," wrote Sens. Harry Reid, Edward Kennedy, Richard Durbin and Russ Feingold. "You may not simply disregard the law."
But as Roberts reports, one Republican senator told CBS News on condition of anonymity she might consider loosening the standards for approving the wiretap and allowing more officials at the Justice Department, not just Attorney General Alberto Gonzales, to authorize eavesdropping, so that it could begin
And the former director of the NSA, U.S. Air Force Gen. Michael Hayden, said the domestic spying program would have likely picked up communications among the 9/11 hijackers.
"Had this program been in effect prior to 9/11, it is my professional judgment that we would have detected some of the 9/11 al Qaeda operatives in the United States," Hayden said.
However, the general did not give any specific evidence for his claim, reports CBS News national security correspondent David Martin, adding that Hayden stopped short of saying warrant-less eavesdropping would have
Before the attacks, head hijacker, Mohammed Atta, exchanged e-mails with Ramzi bin al-Shibh, a key al Qaeda operative in Germany, using a simple code to discuss which targets to hit.
None of those communication were intercepted, not because of legal restrictions on NSA, Martin reports, but because American intelligence did not have a clue either man belonged to al Qaeda.
In addition, Martin adds, two of the hijackers were suspected members of al Qaeda, but the NSA's inability to intercept their calls had
nothing to do with legal restrictions -- they simply slipped the CIA in Asia.
Mr. Bush said the NSA program is limited to communications between the United States and people overseas who are linked to al Qaeda. He said the NSA program has helped prevent terrorist attacks and save American lives, although the government has not given any specifics.
"Officials here learn information about plotters and planners and people who would do us harm," Mr. Bush said, reading from note cards. "Now, I understand there's some in America who say, 'Well, this can't be true there are still people willing to attack.' All I would ask them to do is listen to the words of Osama bin Laden and take him seriously."
However, no one in the political debate over the war on terror or the NSA program has suggested that terrorists no longer want to attack the United States. Rather, Bush's critics have argued that the law requires him to get permission from the Foreign Intelligence Surveillance Court to eavesdrop on communications involving Americans.
Sen. Hillary Rodham Clinton, D-N.Y., issued a blistering attack on Mr. Bush's explanations.
"Obviously, I support tracking down terrorists," she said. "I think that's our obligation. But I think it can be done in a lawful way. 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."
Mr. Bush said he had the legal right to do whatever he could to prevent further attacks and that the NSA program "is fully consistent with our nation's laws and Constitution."
"I'll continue to reauthorize this program for so long as our country faces a continuing threat from al Qaeda and related groups," Mr. Bush said. "This enemy still wants to do harm to the American people. We cannot let the fact that we have not been attacked lull us into the illusion that the threats to our nation have disappeared."