The Justice Department’s effort to shield President Bush from judicial scrutiny for his decision to order warrantless wiretaps is set to face its most difficult challenge. This afternoon in San Francisco, a federal appellate court will hear arguments over whether a lawsuit challenging the wiretapping program involves secrets too sensitive for litigation.

Nearly two years after the disclosure of a National Security Agency program that monitors international communications of terror suspects, the suit on today’s docket is widely considered to have the best chance of drawing the judiciary into a debate over the legality of the program.

Nonetheless, San Francisco’s 9th U.S. Circuit Court of Appeals could dismiss the suit at the Justice Department’s urging. If that is the outcome of today’s hearing, President Bush may escape having any of the federal circuits rule on whether the Terrorist Surveillance Program, as the Justice Department calls it, was within his authority.

“What we’re at risk of here is a tacit approval of the president’s theory of the overarching power of the executive by default,” the lawyer who will argue against the government at today’s appeal, Jon Eisenberg, said in an interview.

So far only one federal judge has issued an opinion on the program’s legality, ordering a halt to the wiretapping. An appellate court in Cincinnati later overturned the decision on the grounds that the plaintiffs could not prove they were actually targeted by the surveillance. If the 9th Circuit dismisses the case before it, legal scholars say the Supreme Court is unlikely to take up the issue as there wouldn’t be any conflict to resolve between the circuits.

Congress’s decision last month to allow President Bush to conduct some wiretaps without court supervision also may relieve some pressure on the Supreme Court to review the matter.

“We may have a situation in which an important legal issue is not resolved in the courts,” a law professor at Duke University, Curtis Bradley, said. “And what if another president later claims the right to engage in surveillance without a warrant? We won’t have any judicial decision to help us.” The suit before the 9th Circuit today is brought by an Oregon branch of an Islamic foundation, which the federal government has designated as a terrorist organization due to alleged financial links with Al Qaeda. The organization, Al-Haramain Foundation, denies the alleged links.

Al-Haramain has an advantage over other plaintiffs — including civil-rights attorneys and journalists who have challenged the wiretap program in other suits. That advantage is that the foundation claims to have seen government documentation proving that communications between its director and two of its lawyers were intercepted. The government accidentally handed over to the organization a document, which the organization says shows just that. A district judge last year in Oregon, Garr King, ordered the return of the document, but agreed that the plaintiffs could cite the document by memory and that the lawsuit could go forward. Because the government maintains that national security requires the NSA not to disclose whom the wiretap program targeted, Al-Haramain is conceivably the only organization presently in a position to prove that it has standing to sue.

The Justice Department says the suit still must be dismissed regardless of the “Sealed Document,” which is “protected by the state secrets privilege,” according to a public Justice Department brief. “Even if the plaintiffs knew that one or more of them were subjected to TSP surveillance,” allowing the suit to go forward would “inflict further harm to national security by publicly confirming or denying that fact,” the Justice Department argues in a brief.

Mr. Eisenberg said that in preparation for today’s argument, he reread the transcript of the famous David Frost interview with President Nixon.

“Nixon said that when the president does it, that means it is not illegal,” Mr. Eisenberg said. “I think what the government is saying here is that when the president does it in secret, that means it is not illegal.”

Justice Department attorney, Greg Garre, will argue for the government today.

The 9th Circuit is also scheduled to hear a second appeal today involving NSA surveillance. The case alleges that AT&T shared phone records with the agency.