In dissent, Justice Alito countered that it would be difficult to limit the sweep of the ruling. He added that the court’s constitutional analysis was flawed. “The real thrust of the majority opinion is that the Constitution is less tolerant of racial bias than other forms of juror misconduct, but it is hard to square this argument with the nature of the Sixth Amendment right on which petitioner’s argument and the court’s holding are based,” he wrote. “What the Sixth Amendment protects is the right to an ‘impartial jury.’ Nothing in the text or history of the amendment or in the inherent nature of the jury trial right suggests that the extent of the protection provided by the amendment depends on the nature of a jury’s partiality or bias.”

In earlier cases, the Supreme Court has said that even egregious misconduct in the jury room cannot be used to challenge a conviction if it would require jurors to testify about what was said there. Until Monday, though, the court had never confronted whether racial or ethnic prejudice requires an exception to the general rule.

In 1987, in Tanner v. United States, the Supreme Court let stand convictions in a mail fraud case in Florida even though the jury had treated the trial as “one big party” fueled by “rampant drug and alcohol abuse,” as one juror described it. During recesses, jurors drank pitchers of beer and liters of wine, and they used marijuana and cocaine.

Justice Sandra Day O’Connor, writing for the majority, said there were good reasons to ignore “irresponsible or improper juror behavior” if it was based on jurors’ accounts of what had gone on in the jury room.

After-the-fact challenges based on jurors’ testimony, she wrote, would make it less likely that jurors would speak candidly during deliberations. Allowing such challenges would encourage lawyers to harass former jurors, she said, and undermine the finality of verdicts.

In 2014, in Warger v. Shauers, the Supreme Court unanimously ruled that jurors may not testify about what went on during deliberations, even to expose dishonesty during jury selection.