''I think most Americans understand the need to find out what the enemy's thinking, and that's what we're doing,'' Bush told reporters in San Antonio as he visited wounded soldiers at the Brooke Army Medical Center.
''They attacked us before, they'll attack us again if they can,'' he said. ''And we're going to do everything we can to stop them.''
Bush's strong defense of the NSA program, which he authorized in 2002 to allow some domestic eavesdropping without court warrants, came as a leading Democratic lawmaker called on the administration to make available current and former high-level officials to explain the evolution of the secret program.
Sen. Arlen Specter, a Pennsylvania Republican and chairman of the Judiciary Committee, has already pledged to make hearings into the program one of his highest priorities.
In a letter to Specter on Sunday, Sen. Charles Schumer, D-N.Y., who is also on the committee, said the panel should also explore ''significant concern about the legality of the program even at the very highest levels of the Department of Justice.''
The New York Times reported Sunday that James Comey, then deputy attorney general, refused to sign on to the recertification of the program in March 2004.
That prompted two of Bush's most senior aides - Andrew Card Jr., his chief of staff, and Alberto Gonzales, then the White House counsel and now the attorney general - to make an emergency hospital visit to John Ashcroft, then the attorney general, to try to persuade him to give his authorization, as required by White House procedures for the program.
Officials with knowledge of the events said that Ashcroft also appeared reluctant to sign on to the continued use of the program, and that the Justice Department's concerns appear to have led in part to the suspension of the program for several months. After a secret audit, new protocols were put in place at the NSA to better determine how the agency established the targets of its eavesdropping operations, officials have said.
Asked Sunday about internal opposition, Bush said: ''This program has been reviewed, constantly reviewed, by people throughout my administration. And it still is reviewed.
''Not only has it been reviewed by Justice Department officials, it's been reviewed by members of the United States Congress,'' he said. ''It's a vital, necessary program.''
But Schumer, in an appearance on ''Fox News Sunday,'' said the White House should have to explain the apparent internal dissent over the program.
''I hope the White House won't hide behind saying 'executive privilege, we can't discuss this,' '' Schumer said. ''That's the wrong attitude.''
The Justice Department said Friday that it had opened an investigation into the disclosure of the NSA program, which was first reported by The New York Times on Dec. 15.
McConnell said of the disclosure, ''This needs to be investigated, because whoever leaked this information has done the U.S. and its national security a great disservice.''
As Bush continued to defend the program in San Antonio, he was asked about a remark he made in Buffalo in 2004 at an appearance in support of the antiterrorism law known as the USA Patriot Act, where he discussed government wiretaps.
''Any time you hear the United States government talking about wiretap,'' Bush said in Buffalo, ''a wiretap requires a court order.''
He added: ''Nothing has changed, by the way. When we're talking about chasing down terrorists, we're talking about getting a court order before we do so.''
Bush also emphasized that the program was ''limited'' in nature and designed to intercept communications from known associates of al-Qaida to the United States.
