A federal judge has resigned from the court that oversees government surveillance in intelligence cases in protest of President Bush's secret authorization of a domestic spying program, according to two sources.

U.S. District Judge James Robertson, one of 11 members of the secret Foreign Intelligence Surveillance Court, sent a letter to Chief Justice John D. Roberts Jr. late Monday notifying him of his resignation without providing an explanation.

Two associates familiar with his decision said yesterday that Robertson privately expressed deep concern that the warrantless surveillance program authorized by the president in 2001 was legally questionable and may have tainted the FISA court's work.

Robertson, who was appointed to the federal bench in Washington by President Bill Clinton in 1994 and was later selected by then-Chief Justice William H. Rehnquist to serve on the FISA court, declined to comment when reached at his office late yesterday.

Word of Robertson's resignation came as two Senate Republicans yesterday joined the call for congressional investigations into the National Security Agency's warrantless interception of telephone calls and e-mails to overseas locations by U.S. citizens suspected of links to terrorist groups. They questioned the legality of the operation and the extent to which the White House kept Congress informed.

Sens. Chuck Hagel (Neb.) and Olympia J. Snowe (Maine) echoed concerns raised by Arlen Specter (R-Pa.), chairman of the Senate Judiciary Committee, who has promised hearings in the new year.

‘A great national debate’
"There's going to be a great national debate on this subject," Specter told reporters yesterday, while emphasizing concerns over the White House's legal arguments in support of the program.

The hearings, possibly in several committees, would take place at the beginning of a midterm election year during which the prosecution of the Iraq war is also likely to figure prominently in key House and Senate races.

Hagel and Snowe joined three Democratic colleagues -- Dianne Feinstein (Calif.), Carl M. Levin (Mich.) and Ron Wyden (Ore.) -- in calling for a joint investigation by the Senate's Judiciary and Intelligence panels into the classified program.

Not all Republicans agreed with the need for hearings and backed White House assertions that the program is a vital tool in the war against al Qaeda.

"I am personally comfortable with everything I know about it, and I'll be watching it as this debate goes on over the next few weeks," Acting House Majority Leader Roy Blunt (R-Mo.) said in a phone interview.

The White House continued to insist yesterday that the classified surveillance program is legal and that key congressional leaders have been informed of the NSA activities since they began shortly after the Sept. 11, 2001, terrorist attacks.

White House spokesman Scott McClellan suggested that the secrecy around the program may prohibit White House cooperation with any congressional investigation. "This is still a highly classified program, and there are details that it's important not be disclosed," McClellan said.

"We've already briefed the leadership and the leaders of the relevant committees," McClellan said, "and the attorney general's going back talking to additional members about this so that they do have a better understanding of this authorization and what it's designed to do and how it is narrowly tailored and limited in how it's used."

Since the program was made public last week by the New York Times, the White House has sparred publicly with key Democrats over whether Congress was fully informed and allowed to conduct oversight of the operation.

The news also spurred considerable debate among federal judges, including some who serve on the secret FISA court. For more than a quarter-century, that court had been seen as the only body that could legally authorize secret surveillance of espionage and terrorism suspects, and only when the Justice Department could show probable cause that its targets were foreign governments or their agents.

Robertson indicated privately to colleagues in recent conversations that he was concerned that information gained from warrantless NSA surveillance could have then been used to obtain FISA warrants. FISA court Presiding Judge Colleen Kollar-Kotelly, who had been briefed on the spying program by the administration, raised the same concern in 2004, and insisted that the Justice Department certify in writing that it was not occurring.

"They just don't know if the product of wiretaps were used for FISA warrants -- to kind of cleanse the information," said one source, who spoke on the condition of anonymity because of the classified nature of the FISA warrants. "What I've heard some of the judges say is they feel they've participated in a Potemkin court."

Considered a liberal judge
Robertson is considered a liberal judge who has often ruled against the Bush administration's assertions of broad powers in the terrorism fight, most notably in Hamdan v. Rumsfeld. Robertson held in that case that the Pentagon's military commissions for prosecuting terrorist suspects at Guantanamo Bay, Cuba, were illegal and stacked against the detainees.

Some FISA judges reached yesterday said they were saddened by the news of Robertson's resignation and wanted to hear more about the president's program.

"I love Jim Robertson and think he's a wonderful guy," said Judge George P. Kazen, another FISA judge. "I guess that's a decision he's made and I respect him. But it's just too quick for me to say I've got it all figured out."

Bush said Monday that the White House briefed Congress more than a dozen times. But those briefings were conducted with only a handful of lawmakers who were sworn to secrecy and prevented from discussing the matter with anyone or seeking outside legal opinions.

Sen. John D. Rockefeller IV (D-W.Va.) revealed Monday that he had written to Vice President Cheney the day he was first briefed on the program in July 2003, raising serious concerns about the surveillance effort. House Minority Leader Nancy Pelosi (D-Calif.) said she also expressed concerns in a letter to Cheney, which she did not make public.

Yesterday, the chairman of the Senate Select Committee on Intelligence, Pat Roberts (R-Kan.), issued a public rebuke of Rockefeller for making his letter public. Roberts's statement did not say whether he would support a joint inquiry with Specter's committee.

In response to a question about the letter, Sen. John McCain (R-Ariz.) suggested Rockefeller should have done more if he was seriously concerned. "If I thought someone was breaking the law, I don't care if it was classified or unclassified, I would stand up and say 'the law's being broken here.' "

But Rockefeller said the secrecy surrounding the briefings left him with no other choice and disputed Roberts's claims that he kept his concerns to himself. "I made my concerns known to the vice president and to others who were briefed. The White House never addressed my concerns," Rockefeller said. He also called for bipartisan hearings.

The Democratic leadership wrote separately to Bush asking him to provide Congress with additional information on the program.