WASHINGTON — In the wake of revelations about incendiary text messages from Virginia Thomas, the wife of Justice Clarence Thomas, to the Trump White House, mostly in the weeks leading up to the Jan. 6 attack, Democrats in Congress urged Chief Justice John G. Roberts Jr. to take action.

But the chief justice is powerless to force Justice Thomas to recuse himself.

The messages, which embraced conspiracy theories about the election and urged Mark Meadows, President Donald J. Trump’s chief of staff, to work to overturn the results, may well mean that Justice Thomas should disqualify himself from cases on the 2020 election and its aftermath.

Indeed, legal experts said that is not a close question. But it is one that the Supreme Court’s norms and structure leave to the justice in question.

His title notwithstanding, the chief justice is not the boss of the other members of the court. He has some administrative responsibilities and the power to assign the majority opinion when he is in the majority. He presides at the impeachment trials of sitting presidents. He makes $12,500 more than his colleagues.

But that basically exhausts the practical differences between the roles of the chief justice and the eight associate justices. Indeed, Chief Justice Roberts has joked about the “odd historical quirk” that gives chief justices only one vote.

He can lead by example, of course. He can make public statements, as when, on different occasions, he rebuked Mr. Trump and Senator Chuck Schumer of New York, the Democratic leader, for attacks on members of the federal judiciary.

But he does not presume to tell other justices what to do. After news reports that he had asked colleagues to wear masks on the bench during the Omicron surge, he took the unusual step of issuing a public statement.

“I did not request Justice Gorsuch or any other justice to wear a mask on the bench,” he said. (Justice Neil M. Gorsuch was the only justice who did not wear a mask at some arguments in January.)

Recusal decisions, the chief justice has said, are also left to individual justices.

“I have complete confidence in the capability of my colleagues to determine when recusal is warranted,” Chief Justice Roberts wrote in 2011, amid calls for Justice Thomas’s recusal from a challenge to the Affordable Care Act, which Ms. Thomas had worked against. “They are jurists of exceptional integrity and experience whose character and fitness have been examined through a rigorous appointment and confirmation process.”

Letting other justices second-guess their colleagues’ recusal decisions would be undesirable, Chief Justice Roberts wrote.

“The Supreme Court does not sit in judgment of one of its own members’ decision whether to recuse in the course of deciding a case,” he wrote. “Indeed, if the Supreme Court reviewed those decisions, it would create an undesirable situation in which the court could affect the outcome of a case by selecting who among its members may participate.”

Speaking at a news conference on Capitol Hill on Tuesday, Speaker Nancy Pelosi, Democrat of California, said she believed Supreme Court justices should face a stricter code of ethics, adding that she had suggested to Democratic lawmakers that they hold a hearing highlighting such a provision in their voting, ethics and campaign finance bill.

“I hear people say from time to time, ‘Well, it’s a personal decision of a judge as to whether he should recuse himself,’” Ms. Pelosi said. “Well, if your wife is an admitted and proud contributor to a coup of our country, maybe you should weigh that in your ethical standards.”

While Chief Justice Roberts may lack formal power over his colleagues, it is at least conceivable that Justice Thomas would consult him on whether the activities of his wife, who goes by Ginni, should require recusal in some cases.

At a 2019 budget hearing before a House committee, Justice Elena Kagan said that when she and other members of the court confronted ethical issues they sometimes sought advice from one another, and especially from Chief Justice Roberts.

“Maybe we’ll consult with our colleagues, or some of them,” she added. “The chief justice in particular.”

Justice Kagan said that “the chief justice is studying the question of whether to have a code of judicial conduct that’s applicable only to the United States Supreme Court.”

“That’s something that we have not discussed as a conference yet,” she said, referring to the justices’ private meetings. “And that has pros and cons, I’m sure. But it’s something that is being thought very seriously about.”

Three years have passed since Justice Kagan spoke, and the code she described has not appeared.

But recusal is already the subject of a federal law that applies to the justices. “Any justice, judge or magistrate judge of the United States shall disqualify himself,” it says, “in any proceeding in which his impartiality might reasonably be questioned.”

That law was the subject of a 1993 letter signed by seven justices. It addressed the question of what to do when a family member had worked on a case in its early stages or was a partner in a law firm representing a client in a case before the Supreme Court.

The seven justices wrote that they had “spouses, children or other relatives” who might fall into those categories. It made sense, they wrote, to announce clear rules about how such conflicts would be addressed before they arose.

“Determining and announcing our policy in advance will make it evident that future decisions to recuse or not to recuse are unaffected by irrelevant circumstances of the particular case, and will provide needed guidance to our relatives and the firms to which they belong,” the justices wrote.

The letter drew some fine distinctions. In general, it said, recusal was not required “by reason of a relative’s participation as a lawyer in earlier stages of the case” unless the relative had been the lead lawyer.

The letter added that the justices who signed it would recuse “from all cases in which appearances on behalf of parties are made by firms in which our relatives are partners, unless we have received from the firm written assurance that income from Supreme Court litigation is, on a permanent basis, excluded from our relatives’ partnership shares.”

The letter is notable not because it covered circumstances like that arising from Ms. Thomas’s text messages, but because it was a careful and transparent public discussion of difficult ethical questions arising from the activities of family members. And because it was signed by Justice Thomas.

Catie Edmondson contributed reporting.