The president said he had not ordered an investigation into who revealed the initiative to The New York Times because he expects the US Justice Department to carry out a formal legal probe.
In a year-end press conference, Mr Bush said he had repeatedly re-authorised the surveillance effort and would do so "for so long as the nation faces the continuing threat of an enemy that wants to kill our American citizens."
"We need to connect the dots before the enemy attacks, not after," he said, brushing off the congressional outcry and calls for an investigation into the initiative's legality, saying he had acted within his wartime powers.
Shameful act
Under a secret order signed by Mr Bush in 2002 that was revealed last week by The New York Times, the National Security Agency (NSA) is permitted to monitor US citizens' telephone and electronic mail when they are in touch with someone overseas without a court warrant, something which is contrary to legal precedent.
"My personal opinion is it was a shameful act for someone to disclose this very important program in time of war," said the president, who struck a forceful tone throughout the hour-long question-and-answer session.
The US leader recalled a 1998 leak to the media, which he said had helped Osama bin Laden evade a US manhunt, and used the episode to back up his claim that the New York Times story could endanger US security.
"The fact that we were following Osama bin Laden because he was using a certain type of telephone, made it into the press as the result of a leak," Mr Bush said during an end-of-year news conference.
"And guess what happened? Osama bin Laden changed his behaviour. He began to change how he communicated."
While he did not identify the newspaper involved, he was apparently referring to a Washington Times report published during the Clinton administration in 1998, which said US spies had listened to phone calls by Al-Qaeda leaders.
Authority from Congress
Mr Bush said his authority to carry out the program derived from the US Constitution and the US Congress's vote to authorise the use of force in Afghanistan to wipe out Al-Qaeda after the September 11, 2001 attacks.
But he flatly rejected charges that he had kept the US Congress in the dark about the initiative's true extent and that his justification for the secret program amounted to laying claim to "unchecked power" in wartime.
Patriot act
As debate on civil liberties and the battle against terrorism rumbled on, Mr Bush demanded that senators drop their opposition to renewal of the Patriot Act, which has also attracted concern from the civil liberties lobby.
"In the war on terror, we cannot afford to be without this law for a single moment."
The White House has previously said Mr Bush would not sign a temporary extension of the legislation, something some lawmakers have proposed for the duration of the debate.