WASHINGTON — The Supreme Court on Monday ruled that prosecutors in Georgia violated the Constitution by striking every black prospective juror in a death penalty case against a black defendant. The vote was 7 to 1, with Justice Clarence Thomas dissenting.

The case, Foster v. Chatman, arose from the 1987 trial of Timothy T. Foster, an African-American facing the death penalty for killing Queen Madge White, an elderly white woman, when he was 18.

In notes that did not surface until decades later, prosecutors marked the names of black prospective jurors with a B and highlighted those names in green. They circled the word “black’’ where potential jurors had noted their race on questionnaires.

They ranked the black prospective jurors in case “it comes down to having to pick one of the black jurors,’’ as the prosecution’s investigator put it in a draft affidavit at the time. In the end, prosecutors struck all four black potential jurors.

After Foster was convicted, Stephen Lanier, the lead prosecutor, urged the all-white jury to impose a death sentence to “deter other people out there in the projects.’’

The jury did so.

Chief Justice John G. Roberts Jr., writing for the majority, said the prosecutors had violated a 1986 decision, Batson v. Kentucky, in which the Supreme Court ruled that race discrimination in jury selection was unconstitutional and required lawyers accused of it to provide a nondiscriminatory explanation.

That is a forgiving standard. “All I have to do is have a race-neutral reason,’’ Lanier said at the time, “and all of these reasons that I have given the court are racially neutral.’’

But Roberts rejected several of Lanier’s reasons, calling them pretextual. The chief justice focused on two prospective jurors: Marilyn Garrett and Eddie Hood.

Lanier had offered a list of 11 reasons for striking Garrett, including that she was too young.

“Yet Garrett was 34,’’ Roberts wrote, “and the state declined to strike eight white prospective jurors under the age of 36. Two of those white jurors served on the jury; one of those two was only 21 years old.’’

Lanier also said Garrett was unfit to serve because she was divorced. But, the chief justice wrote, Lanier “declined to strike three out of the four prospective white jurors who were also divorced.’’

Lanier gave eight reasons for striking a second prospective juror, Hood, including that his son was the same age as the defendant and had been convicted of a crime that was, he said, “basically the same thing that this defendant is charged with.’’

Roberts called this “nonsense.’’

“Hood’s son had received a 12-month suspended sentence for stealing hubcaps from a car in a mall parking lot five years earlier,’’ he wrote. “Foster was charged with capital murder of a 79-year-old widow after a brutal sexual assault.’’

Lanier also said he doubted that Hood would impose the death penalty in light of his religious faith. “But the record persuades us that Hood’s race, and not his religious affiliation, was Lanier’s true motivation,’’ Roberts wrote.

In sum, the chief justice wrote, “we are left with the firm conviction that the strikes of Garrett and Hood were motivated in substantial part by discriminatory intent.’’

In other action Monday:

¦ The Supreme Court left in place a court-imposed congressional redistricting map in Virginia, dismissing a challenge from three Republican representatives.

The court’s unanimous decision said the members of Congress had not shown that they had suffered the sort of direct and concrete injury that gave them standing to sue.

¦ The justices made it easier for federal workers to file employment discrimination lawsuits after quitting their jobs over conditions they consider intolerable.

They ruled, 7 to 1, that workers who bring so-called ‘‘constructive discharge’’ claims have 45 days from the time they resign to begin the process. The court rejected the Justice Department’s position that the clock should start running when the alleged abuse occurs.

¦ The high court said it won’t reconsider its decision to reject former Illinois governor Rod Blagojevich’s appeal of his corruption convictions.