The U.S. Supreme Court will confer Thursday on the execution of condemned Florida killer John A. Spenkelink.

His electrocution was stopped just seven hours short of death by Supreme Court Justice Thurgood Marshall after two other justices had refused to stay the execution.

Marshall, who is known to oppose the death penalty on principle, declined to discuss his order in detail but said, "Given the Court of Appeals' divided vote on whether to grant a certificate of probable cause, the irrevocable nature of the penalty to be imposed and the ability of the full court to consider this case within 36 hours . . . I believe it appropriate to grant the application for a stay until further action by the entire court."

Thursday is the High Court's regular day for conferences and a court spokeswoman confirmed that the Spenkilink case is on the agenda. This will be Spenkelink's fifth appeal to the high court.

In Atlanta, Judge Elbert P. Tuttle of the 4th U.S. Circuit Court of Appeals in New Orleans also signed a stay order "until further order of the court."

Tuttle said the basis for Spenkelink's appeal to his court was "an allegation of inadequate and incompetent counsel. That's really all I can say right now." The court had rejected a previous Spenkelink petition for a stay of execution by a 2-to-1 decision - the "divided vote" cited by Justice Marshall.

Tuesday, a federal judge in Tampa had granted a stay of execution for another condemned man, Willie J. Darden, and ordered a hearing on charges that the prosecutor in Darden's trial had made inflammatory and intemperate remarks to the jury. Darden was convicted in 1974 of the robbery-murder of a Lakeland furniture store owner.

Both death warrants expire Friday at noon and cannot be extended. If the courts do not order the executions canceled, Gov. Robert D. Graham will have to sign new death warrants before the executions can proceed.

Florida Attorney General Jim Smith said he will fly to Washington, Atlanta and New Orleans to argue the state's case in the Spenkelink and Darden appeals. Smith said the courts should allow the executions to proceed before the death warrants expire.

Graham's chief counsel, Robin Gibson, met with reporters in Tallahassee but refused to discuss the governor's feelings concerning the executions. He and Graham's press secretary Steve Hull, agreed that the governor showed no outward sign of emotion when notified of the last-minute stays.

Graham walked to the iron-barred fence surrounding the mansion grounds and spoke briefly with demonstrators who maintained an allnight vigil there. Gibson and Hull said the governor felt that the demonstrators were entitled to hear the news. He was unaware that the demonstrators had already been told of the stay of Spenkelink's execution.

According to Hull, about 1,000 telephone calls came into the governor's office today when word of the stays got around. He said 320 of the callers opposed the death penalty. Graham, who took office in January this year, campaigned in favor of the death penalty when he was running for governor and had supported it when he served in the state Senate.

Hull said, "He believes it has a deterrent effect." CAPTION: Picture, The Rev. Joe Ingle drives John Spenkelink's mother, Lois, away from a meeting with her son in prison. AP