The Bush administration is launching a campaign to go on the offense against criticism of its wiretapping program.

As Congress prepares to hold hearings into whether President Bush has the legal authority to authorize the National Security Agency to intercept overseas communications of people in the United States, Bush on Wednesday will visit the ultra-secret NSA.

The administration argues that the president has the constitutional authority to let intelligence officials listen in on international phone calls of Americans with suspected ties to terrorists.

"We are stepping up our efforts to educate the American people," White House spokesman Scott McClellan said about Bush's trip to the NSA, based at Fort Meade in Maryland.

"This is a critical tool that helps us save lives and prevent attacks," he said. "It is limited and targeted to Al Qaeda communications, with the focus being on detection and prevention."

Meanwhile, Sen. Patrick Leahy, D-Vt., on Friday introduced a resolution saying Congress did not authorize the NSA program when it passed that 2001 resolution governing the use of military force in the War on Terror.

"Now that the illegal spying of Americans has become public and the president has acknowledged the four-year-old program, the Bush administration's lawyers are contending that Congress authorized it," said Leahy, the ranking Democrat on the Judiciary Committee and a co-author of the original USA Patriot Act. "The September 2001 Authorization to Use Military Force did no such thing."

On Monday, deputy national intelligence director Mike Hayden, who headed the National Security Agency when the program began in October 2001, will speak on the issue a the National Press Club.

On Tuesday, Attorney General Alberto Gonzales will deliver a speech on the program in Washington. Gonzales plans to testify publicly about the secret program at a Senate hearing set to begin Feb. 6.

Gonzales said he reached an agreement with Sen. Arlen Specter, R-Pa., chairman of the Senate Judiciary Committee, to answer questions about the legal basis — but not the operations — of the NSA's warrantless eavesdropping on telephone conversations between suspected terrorists and people in the United States.

Gonzales this week sent congressional leaders a 42-page legal defense of warrantless eavesdropping, expanding on arguments that he and other administration officials have been making since the program was first disclosed last month.

The memo argues that Bush has authority to order the warrantless wiretapping under the Constitution and the post-Sept. 11 congressional resolution granting him broad power to fight terrorism.

Vice President Dick Cheney, who was to meet with congressional leaders at the White House on Friday to discuss the issue, defended the program on Thursday in New York in a speech to the conservative Manhattan Institute. He stressed that the program was limited and conducted in a way that safeguards civil liberties.

At a briefing held by House Democrats on Friday, the American Civil Liberties Union called the program an illegal operation.

"The executive power of our country is not an imperial power," Caroline Fredrickson, the director of the ACLU legislative office in Washington, told Democratic members of the House Judiciary Committee.

"The president has demonstrated a dangerous disregard for our Constitution and our laws with his authorization for this illegal program," she said.

Fredrickson spoke to Democratic members of the House Judiciary Committee. The ACLU filed suit against the NSA on Jan. 17 on behalf of journalists, nonprofit groups, terrorism experts and community advocates. The suit alleges that the NSA program violates the First and Fourth amendments and the separation of power.

Some Democratic leaders on Thursday sent a letter to Cheney asking that any future consultations regarding the NSA program be discussed with all members of the Senate and House intelligence committees. They also want the administration to work with lawmakers on ways to better keep Congress informed on "vital national security matters."

"We strongly support intelligence operations, consistent with our laws, to prevent acts of terrorism and recognize the need to keep these operations secret. Yet, the framers of the Constitution clearly intended that the executive branch be held accountable to the Congress and the American people for its actions," the group wrote.

Senate Democratic Leader Harry Reid of Nevada, House Democratic Leader Nancy Pelosi of California, along with Sen. John D. Rockefeller, D-W.Va., and Rep. Jane Harman, D-Calif. -- the ranking Democrats on the Senate and House intelligence committees, respectively -- wrote that the administration has limited the information provided to Congress and that even those briefings that were provided were limited.

"It has been our experience that many of the restricted notifications on intelligence matters that we have received from your Administration have been on precisely the sort of issues that would have benefited from the scrutiny of the full membership of the House and Senate Intelligence Committees."

The Associated Press contributed to this report.