ASHINGTON, Dec. 8 — Delma Banks Jr., a Texas death row inmate, lay strapped to a gurney barely 10 minutes before execution last March when the Supreme Court granted him a stay in order to hear his claim that prosecutorial misconduct rendered his murder conviction and death sentence unconstitutional.

Mr. Banks, the longest-serving of 453 prisoners on Texas' death row, has had 15 execution dates in the nearly 24 years since he was sentenced to die at age 20 for killing a 16-year-old acquaintance and stealing his car. Given the justices' reactions on Monday to the arguments in his Supreme Court appeal, it is unlikely that he will face an execution date again in the near future.

The court appeared strongly inclined to set aside at least the death sentence, if not the conviction itself, in a case that death penalty opponents have cited as an example not only of what can go wrong in a capital prosecution but also of what they deem the casual attitude that the federal appeals court with jurisdiction over Texas has adopted toward the capital cases it reviews.

The Federal District Court in Texarkana granted Mr. Banks a writ of habeas corpus and overturned his death sentence, but the United States Court of Appeals for the Fifth Circuit, in New Orleans, reversed that decision with a 78-page opinion marked "not for publication," a designation most courts reserve for cases that involve straightforward applications of law with little consequence to anyone beyond those directly involved.

In Mr. Banks's habeas corpus petition, his lawyers demonstrated that the prosecution had withheld from the defense and the jury the knowledge that its main witness, who gave damaging evidence at the sentencing hearing, was a paid police informant. The defense maintained that this knowledge would have substantially undermined the man's credibility.

Another main prosecution witness lied when he said he had spoken to no one in advance about his courtroom testimony. That lie was uncovered only in 1999, when Mr. Banks's defense team obtained a transcript of the witness's pretrial interview with law enforcement officials, in which he was coached in what to say.

During Monday's arguments, the state's lawyer, Gena A. Bunn, chief of the Texas attorney general's capital litigation division, acknowledged errors in the prosecution's handling of the case but said they were not "material," because, she maintained, they would not have led to a different verdict had they been known to the jury. In any event, she said, the Banks defense team failed to uncover and present evidence of those errors in a timely manner. "There was an obligation for them to pursue their claims further," she said.

Several justices challenged Ms. Bunn on those points. "Why wasn't it the obligation of the prosecution, having deceived the jury and the court, to come clean?" Justice Ruth Bader Ginsburg asked Ms. Bunn. "I just don't understand," Justice Ginsburg continued. "The prosecution was best situated to know what happened."

Justice Stephen G. Breyer took up the challenge. "What bothers me about your position," he told Ms. Bunn, "is that if we were to say that defense counsel behaves unreasonably if he relies on the prosecution, that's to say that the justice system lacks integrity, and indeed it might contribute to that lack of integrity."

One of the witnesses, Robert Farr, answered "no" during the trial to the question "Have you ever taken any money from some police officers?" In fact, the police had paid him $200 to assist them in locating the gun that was linked to the crime. Suspecting that Mr. Farr was an informer, the Banks lawyers included such an allegation in their habeas corpus petition, to which the state responded by denying all allegations in the 145-page petition.

"So the prosecution can lie and conceal, and the defense still has the burden to discover the evidence?" Justice Anthony M. Kennedy asked in a tone of incredulity.

Even Justice Antonin Scalia, who challenged Mr. Banks's lawyer, George H. Kendall, on several points and tried to be helpful to Ms. Bunn, appeared persuaded that the appeals court's decision could not stand. Justice Scalia portrayed as at least doubtful the appeals court's ruling on an important procedural point: that the prosecution's failure to disclose the existence of the 74-page pretrial-interview transcript could not be challenged at the habeas corpus stage because it had not been part of the initial appeal. "There are a lot of questions on that part of the holding," he said.

The case, Banks v. Dretke, No. 02-8286, reached the Supreme Court against a backdrop of increasing concern about the death penalty in Texas, which has accounted for more than one-third of all executions in the country since 1976, including 23 of 63 this year.

Earlier this year, with only Justice Clarence Thomas dissenting, the court ordered a new sentencing hearing for another Texas death row inmate, Thomas Miller-el, and rebuked the Fifth Circuit, describing it as overly deferential to the state courts. "Deference does not imply abandonment or abdication of judicial review," Justice Kennedy said in that decision, Miller-el v. Cockrell.

In the Banks case, the justices may have been persuaded to issue the last-minute stay by the intervention of a group of retired federal and state judges and prosecutors, who filed an unusual "friend of the court" brief in support of the stay application. The group subsequently filed a more extensive brief supporting the appeal on its merits.

Those who signed the brief included Harold R. Tyler Jr., a former federal district judge who was deputy attorney general during the administration of President Gerald R. Ford; William S. Sessions, former director of the Federal Bureau of Investigation and also a former judge; and John J. Gibbons, a retired chief judge of the United States Court of Appeals for the Third Circuit, to which he was appointed by President Richard M. Nixon.

"We are not a bunch of left-wing kooks trying to undermine the social order in the state of Texas," Judge Gibbons said last week.