If Sarmina's stay is not reversed by the Supreme Court - and Williams' death sentence is reaffirmed - Gov. Corbett would have to sign a new warrant setting a new execution date.
State Supreme Court spokesman Art Heinz said he had no information about how the court might handle the emergency appeal before Wednesday.
It's virtually new terrain for the Supreme Court and governor. Pennsylvania has executed three people since it reinstituted capital punishment in 1978 - the last in 1999 - and only after the individuals gave up appeals and asked to die.
The last contested execution in which last-minute appeals were an issue for the state Supreme Court and governor was in 1962.
Williams' lawyers called on the prosecutor to "stop trying to execute Terry Williams."
"It is deeply disconcerting that the district attorney has filed an appeal," said Shawn Nolan, a member of the defense team from the Federal Defender's death-penalty unit.
"It is legally and ethically unconscionable that Seth Williams and his assistants continue to advocate for the execution of Terry Williams after hiding significant evidence for 28 years from defense counsel, jurors, the courts, the Board of Pardons, and the citizens of Pennsylvania."
Seth Williams, in an afternoon news conference, insisted he was not an unquestioning death-penalty advocate. He said only one Philadelphian had been sentenced to death since he took office in 2010.
"This is about process, and I'll not walk away from doing my job, and that means preserving the integrity of this jury's verdict," Williams said.
Sarmina's ruling came after two days of testimony about whether the 1986 trial prosecutor misled the jury, withholding evidence that might have made it more likely for the jury to sentence Williams to death instead of life.
Sarmina, 59, a former city prosecutor and a judge since 1998, has been a homicide judge for six years and presided over three trials that ended in death sentences.
In an 45-minute opinion delivered before a packed courtroom, Sarmina criticized the trial prosecutor for "gamesmanship" and withholding mitigating evidence the defense attorney could have used in arguing against a death sentence.
Sarmina said the prosecutor, Andrea Foulkes, now a federal prosecutor in Philadelphia, withheld evidence of the sexual proclivities of Williams' victim Amos Norwood and the extent of the prosecution's deal with admitted accomplice Marc Draper.
The withheld evidence, Sarmina said, undermined confidence in the fairness of Williams' 1986 death sentence and made a stay of execution necessary.
Sarmina did affirm the guilty verdict against Williams in Norwood's murder. She said the District Attorney's Office could conduct a new penalty hearing for Williams if it wished to seek execution, or could let Williams, 46, spend the rest of his life in prison without chance of parole.
Through a spokeswoman, Foulkes declined to comment.
Seth Williams and federal prosecutors vigorously defended Foulkes.
"She left this office after a distinguished career here and then carried on with the U.S. Attorney's Office, practicing before federal judges and always with an unblemished record," Williams told reporters.
First Assistant U.S. Attorney Louis Lappen called Foulkes "an outstanding prosecutor with an impeccable record for integrity, professionalism, and dedication to public service."
In testimony last week before Sarmina, Foulkes insisted that she did not withhold evidence. She said the information she had about the victim, a 56-year-old Germantown church volunteer who Terrance Williams says sexually abused him beginning at age 13, was nothing more than "bits and pieces."
Sarmina, however, cited Foulkes' testimony that she suspected Norwood had sex with teenage boys and had a sexual relationship with the then-18-year-old Williams, a Cheyney University freshman and football player.
"She was able to connect all the dots," Sarmina said. "Had a reasonable defense counsel been given all the dots, he also could have connected them."
As it happened, Sarmina said, defense attorney Nicholas Panarella met with Williams one day before his 1986 murder trial began.
Panarella presented no mitigating evidence to try to dissuade the jury from sentencing Williams to death. In Williams' plea for clemency, defense lawyers presented statements from five trial jurors saying they would have sentenced Williams to life had they known about evidence of Norwood's sexual proclivities and abuse of Williams.
Sarmina said Foulkes gave Williams' 1986 defense lawyer statements from Norwood's widow and minister that portrayed him as a kindly, religious man with a reputation for helping poor youths.
Sarmina said the prosecution omitted from Mamie Norwood's statement details on her husband's purported abduction by teenage boys he picked up in his car one night. Norwood's widow said her husband paid the boys $20 in cash and the family stereo system as ransom, and told her not to call police.
And the judge said a statement from the Rev. Charles Poindexter, Norwood's pastor at St. Luke's Episcopal Church, omitted a complaint Poindexter received several years earlier that Norwood had touched the genitals of a 17-year-old acolyte.
"They were sanitized statements," Sarmina said, adding that Williams' attorney could have used the information to portray Norwood as an "unsympathetic victim."
Those comments irked prosecutors, who said the information was not "exculpatory material" that the U.S. Supreme Court has said prosecutors were required to disclose to the defense.
"We're not required to transfer all information," said Deputy District Attorney Ronald Eisenberg, who argued the appeal before Sarmina.
"These are little tidbits of information suggesting, perhaps, that Amos Norwood had an interest in homosexual activities," Eisenberg added.
Eisenberg said there was no independent evidence that Norwood had molested Williams, and that Williams did not make that argument at trial. Instead, Williams testified that he did not know Norwood and was not there when he was killed.
Contact Joseph A. Slobodzian at 215-854-2985, firstname.lastname@example.org, or @joeslobo on Twitter.