Noting that most of the successes at the National Security Agency are never acknowledged, President Bush Wednesday tried to boost morale of the agency's employees, many of which are carrying out his controversial electronic eavesdropping program.

Behind closed doors with employees of the super-secretive agency, which traditionally conducts only overseas surveillance, Bush said the NSA's work is vital to protecting the homeland and to ongoing operations in Afghanistan and Iraq.

He later told the press that he would continue with the program so long as a terrorist threat existed.

"Listen to the words of Usama bin Laden and take them seriously when he says he's going to hurt the American people again, or try to, he means it," Bush told reporters. "I take it seriously and the people of NSA take it seriously and most of the American people take it seriously, as well."

After Sept. 11, 2001, Bush authorized the NSA to conduct some surveillance without warrants of people inside the United States. The wiretaps are only used when someone inside this country is one of the people taking part in an overseas communication with a suspected link to Al Qaeda.

While critics call it a domestic spying program, the White House calls it a "terrorist surveillance program." After receiving harsh criticism from civil liberties groups and Democrats when news of the program was leaked by The New York Times, defending the program is a top priority of the Bush administration.

Bush said part of the terrorists' strategy is to place operatives inside the United States to blend in with the civilian population as they receive orders from overseas.

"We must be able to quickly detect when someone linked to Al Qaeda communicates with someone inside of America," he said, adding that the information gleaned from electronic surveillance has helped prevent attacks and save lives. "We must understand what's going on if we are going to do our job and protect the American people."

The president also stressed that numerous safeguards are in place to ensure the program is not abused or unduly infringes on Americans' rights, and said it is just one of the tools being used to protect the country.

"These tools include surveillance to protect and prevent further attacks by our enemies," Bush said. "I have the authorization both from the Constitution and the Congress to undertake this vital program. The American people expect me to protect our lives and our civil liberties and that's exactly what we're doing with this program."

The Bush administration argues that authority was given to the president in both the Constitution and the congressional resolution passed after the Sept. 11, 2001, attacks, to go to war against terrorists who want to do harm to the United States.

"This enemy still wants to do harm to the American people," Bush said. "We cannot let the fact that we have not been attacked lull us into the conclusion that the terrorists have disappeared. They have not disappeared."

Dems: We Want More Information

But Democrats, including Sen. Hillary Rodham Clinton, said they think the president's explanations do not adequately defend the decision to authorize wiretaps inside U.S. borders.

"Obviously, I support tracking down terrorists. I think that's our obligation. But I think it can be done in a lawful way," said Clinton, a potential 2008 presidential candidate.

"Their argument that it's rooted in the authority to go after Al Qaeda is far-fetched," she said in an apparent reference to a congressional resolution passed after the Sept. 11, 2001, terrorist attack.

Bush's visit is part of an orchestrated, coordinated campaign by the White House to bolster public support for the program. NSA and administration officials argue that the program has been a vital tool in preventing more attacks on the homeland and that it is a good solution to the outdated FISA court, which traditionally has been used to authorize wiretap warrants under the Foreign Intelligence Surveillance Act.

"The president not only has the authority to do what he's doing, but he has the responsibility to do what he is doing, because it's about saving lives. It's about preventing attacks," said White House spokesman Scott McClellan. "It's very limited in nature. And it's focused on international communications involving Al Qaeda members or affiliated terrorist organizations from either communicating inside the United States to someone outside, or communicating from outside the United States to someone inside."

Headquartered in Fort Meade, Md., the president last visited the NSA in June 2002, when the agency was already conducting this monitoring program. The administration says those warrantless wiretaps were very limited in number and very appropriate. Although the White House says it briefed key members of Congress on the program, after The New York Times broke the story, some Democrats came forward to say not enough of them were informed.

Gen. Michael Hayden, who ran the NSA at the time of the surveillance program's launch, this week defended the program, saying investigators were able to target suspected communications if they had "reason to believe" that someone is connected to Al Qaeda, rather than the "probable cause" standard required by the FISA court. The difference, he said, is that the detection methods don't target individuals but trap communications. The probable cause standard, on the other hand, relates to individuals.

Hayden, who now serves as principal deputy director of national intelligence, made clear NSA has often used FISA, especially after Sept. 11, but that in some cases, it is not as effective.

"The purpose of all of this is not to collect reams of intelligence, but to detect and prevent attacks. The intelligence community has neither the time, the resources nor the legal authority to read communications that aren't likely to protect us. And NSA has no interest in doing so. These are communications that we have reason to believe are Al Qaeda communications," Hayden said.

Attorney General Alberto Gonzales on Tuesday also defended the program during a speech at Georgetown University.

"It is an early warning system with only one purpose: to detect and prevent the next attack on the United States," Gonzales said. "It is imperative for national security reasons that we can detect reliably, immediately and without delay whenever communications associated with Al Qaeda enter or leave the United States."

Bush and key members of his administration have recently stepped up public appearances to respond to outcries over the program.

The Senate Judiciary Committee will open hearings Feb. 6 into the program and Gonzales has agreed to testify.

Sen. John McCain said Wednesday he's eager to learn more about the NSA program.

Appearing on NBC's "Today" show, the Arizona Republican said that "no one wants us not to be able to eavesdrop on a conversation, listen in on a conversation between an Al Qaeda operative and anyone else that could be a threat to the United States."

Asked if he thought Bush had broken the law, McCain replied, "I don't know. I want to be perfectly clear. I don't know the answer. That's why I welcome the hearings."

Former Vice President Al Gore, Bush's opponent in the hotly contested 2000 election that ended up being settled by the Supreme Court, charged earlier this month that Bush has violated federal laws.

FOX News' Greg Kelly and The Associated Press contributed to this report.