WASHINGTON (Reuters) - President George W. Bush headed toward a showdown with the Senate over his domestic spying program on Thursday after lawmakers approved subpoenas for documents the White House declared off-limits.

President Bush tours the control room at Brown's Ferry Nuclear Plant in Athens, Alabama, June 21, 2007. REUTERS/Kevin Lamarque

“The information the committee is requesting is highly classified and not information we can make available,” White House spokesman Tony Fratto said in signaling a possible court fight.

The Senate Judiciary Committee approved the subpoenas in a 13-3 vote following 18 months of futile efforts to obtain documents related to Bush’s contested justification for warrantless surveillance begun after the September 11 attacks.

Three Republicans joined 10 Democrats in voting to authorize the subpoenas, which may be issued within days.

“We are asking not for intimate operational details but for the legal justifications,” said Committee Chairman Patrick Leahy, a Vermont Democrat. “We have been in the dark too long.”

Authorization of the subpoenas set up another possible courtroom showdown between the White House and the Democratic-led Congress, which has vowed to unveil how the tight-lipped Republican administration operates.

Last week, congressional committees subpoenaed two of Bush’s former aides in a separate investigation into the firing last year of nine of the 93 U.S. attorneys.

Bush could challenge the subpoenas, citing a right of executive privilege his predecessors have invoked with mixed success to keep certain materials private and prevent aides from testifying.

Bush authorized warrantless surveillance of people inside the United States with suspected ties to terrorists shortly after the September 11 attacks. The program, conducted by the National Security Agency, became public in 2005.

WARTIME POWERS

Critics charge the program violated the 1978 Foreign Intelligence Surveillance Act, which requires warrants. Bush said he could act without warrants under wartime powers.

In January, the administration abandoned the program and agreed to get approval of the FISA court for its electronic surveillance. Bush and Democrats still are at odds over revisions he wants in the FISA law.

“The White House ... stubbornly refuses to let us know how it interprets the current law and the perceived flaws that led it to operate a program outside the process established by FISA for more than five years,” Leahy said.

Interest in the legal justification of the program soared last month after former Deputy Attorney General James Comey testified about a March 2004 hospital-room meeting where then-White House counsel Alberto Gonzales tried to pressure a critically ill John Ashcroft, then the attorney general, to set aside concerns and sign a presidential order reauthorizing the program.

With top Justice Department officials threatening to resign, Bush quietly quelled the uprising by directing the department to take steps to bring the program in line with the law, Comey said.

Leahy noted that when Gonzales, now attorney general, appeared before the panel on February 6, he was asked if senior department officials had voiced reservations about the program.

“I do not believe that these DoJ (department) officials ... had concerns about this program,” Leahy quoted Gonzales as saying. Leahy added, “The committee and the American people deserve better.”