The Senate Judiciary Committee was to question Mr Gonzales about the warrantless wiretaps by the National Security Agency, amid lingering questions about the legality and effectiveness of the program.
A leading member of President George W. Bush's Republican Party said on Sunday that his top question for Gonzales, the president's former White House counsel, will be why the administration has not sought court warrants to eavesdrop, as stipulated by a 1978 law.
Meanwhile, a report by The Washington Post said the NSA eavesdropping program authorised by Mr Bush shortly after the September 11, 2001 attacks has monitored the overseas communications of thousands of Americans.
The program has ignited a political firestorm because it has been conducted without court warrants and has sparked fears that Americans' civil liberties are being abused.
Mr Gonzales is expected to lay down a fierce defense of the program and will argue it is vital to the president's "war on terror" against Osama bin Laden's Al-Qaeda network, according to political observers.
Former NSA head and now the deputy director of national intelligence, General Michael Hayden, said on Sunday that the eavesdropping is critical to protecting the United States from further terrorist attacks.
"It's about speed. It's about hot pursuit of Al-Qaeda communications," he told the American television network ABC News.
General Hayden said "knowing what this program can do, knowing the reality prior to 9/11, ... if we had had this program in place, we would have identified some of the Al-Qaeda operatives in the United States," prior to the 2001 airborne attacks on New York and Washington.
Senator Arlen Specter, the chairman of the powerful Senate Judiciary
Committee, told CBS News Sunday his first question to Gonzales will ask why the Bush administration did not seek court warrants for its eavesdropping.
Mr Bush has argued he has the power under the US Constitution as president to approve the warrantless eavesdropping without a court warrant.
But Senator Specter appeared to see the program's legality in a different light.
"There is a specific statute on the books, the Foreign Intelligence
Surveillance Act (FISA), which says flatly that you can't undertake that kind of surveillance without a court order," Senator Specter said.
The FISA law allows the government to conduct eavesdropping without a court warrant for a window of 72 hours, after which it must seek the authorisation of a special FISA court.
Senator Specter said he had asked Mr Gonzales in a letter why this has not been done, and that Mr Gonzales "wasn't entirely responsive", but that the attorney general did argue it would have resulted in surveillance delays.
Hours before Monday's hearing, The Washington Post reported that the top secret program, run out of NSA's headquarters just outside of Washington, has monitored the overseas phone calls of thousands of Americans, most of whom were later ruled out as potential terrorist suspects.
Two "knowledgeable sources" told the Post that the NSA has monitored the communications of thousands of Americans in the past four years. One source told the newspaper the dragnet had included about 5,000 people.
"Computer-controlled systems collect and sift basic information about hundreds of thousands of faxes, e-mails and telephone calls into and out of the United States before selecting the ones for scrunity by human eyes and ears," the report, citing current and former intelligence officials, said.
However, it noted that most of the recorded communications are quickly dismissed by intelligence analysts as unthreatening.
General Hayden, speaking on Fox News Sunday, declined to confirmed the size of the program, stressing it is designed to target Al-Qaeda operatives and suspects and not law-abiding Americans.
"One of the primary purposes of it is to gather foreign intelligence,"
Hayden stressed, adding that "an analyst ... with all the facts available to him or her at the time, has (to have) cause to believe that one or both of these communicants are Al-Qaeda or Al-Qaeda affiliates". Monday's hearing is due to start at 9:30 pm (1430 GMT).