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.