In a letter to Bush, Rep. Jane Harman, D-Calif., said the National Security Act requires the heads of the various intelligence agencies to keep the entire House and Senate intelligence committees "fully and currently informed of the intelligence activities of the United States."

Only in the case of a highly classified covert action can the president choose to inform a narrower group of Congress members about his decision, Harman said. That action is defined in the law as an operation to influence political, economic or military conditions of another country.

"The NSA program does not qualify as a 'covert action,'" Harman wrote.

Bush and his senior national security aides have said that appropriate members of Congress were briefed more than a dozen times about the National Security Agency's domestic surveillance operations, which Bush first approved the month after the attacks of Sept. 11, 2001.

The highly classified sessions are known to include the "Gang of Eight," which is made up of the top Republican and Democrat in the House and Senate and on the House and Senate Intelligence Committees.

"We believe that Congress was briefed appropriately," White House spokeswoman Dana Perino said Wednesday in response to Harman's letter.

Responding in writing to Harman, House Intelligence Chairman Peter Hoekstra, R-Mich., said Harman had never previously raised concerns about the number of people briefed on the program.

"In the past, you have been fully supportive of this program and the practice by which we have overseen it," he wrote. "I find your position now completely incongruent."

Many details about the scope of electronic surveillance program remain unknown. However, Bush and his aides have asserted the monitoring _ without court warrants _ is narrowly targeted to eavesdrop on calls and e-mails of people who are inside the United States and suspected of communicating with al-Qaida or its affiliates.

Vice President Dick Cheney said Wednesday that the program helped to prevent possible terrorist attacks against the American people: "This program is critical to the national security of United States."

Democrats who have been briefed on the program have raised serious concerns about its legality, but not called for its immediate halt. Republicans and Democrats alike have called for hearings this year.