Advocacy groups have urged the Senate Judiciary Committee to call Supreme Court Justice Clarence Thomas to testify about his relationship with a billionaire GOP donor, a move experts say would escalate conflict between two branches of government and could lead to a separation-of-powers showdown.

Senate Judiciary Chair Richard J. Durbin of Illinois and other Democrats on the committee instead have put the onus on the Supreme Court to address the conduct at the center of two ProPublica reports that Thomas did not disclose luxury trips or a real estate transaction with Harlan Crow.

Durbin said the committee would hold a hearing “regarding the need to restore confidence in the Supreme Court’s ethical standards.” But he has called on Chief Justice John G. Roberts Jr. to do an investigation, a hint of the underlying political and separation-of-powers concerns at play.

And Sen. Sheldon Whitehouse, D-R.I., who already had proposed a bill that would address Supreme Court ethics and transparency, said the U.S. Judicial Conference should refer Thomas to the Justice Department for potential violations of ethics laws.

But there are calls for more action. Gabe Roth, the executive director of Fix the Court, a nonpartisan group that has pushed for transparency at the Supreme Court, called for Thomas to testify before Congress and said lawmakers have the power to subpoena him.

“All parties here, including Justice Thomas, have a stake in rebuilding public confidence in the Court’s integrity, and that can only happen once we have a complete picture of what’s transpired,” Roth said.

Roth also acknowledged that testimony would be easier to get from Crow and others, since “it’s unlikely Justice Thomas would comply with a subpoena, and we’d lose precious years in an ensuing separation-of-powers battle.”

Common Cause and an array of liberal-leaning advocacy groups also called for testimony. The New York Times editorial board on Friday said the Judiciary Committee should call on Thomas to testify if Roberts doesn’t conduct an investigation.

Representatives for Durbin and Whitehouse did not respond to questions about whether they would call on Thomas to testify before the Senate.

Rare event

It’s rare for Supreme Court justices to testify before Congress voluntarily, and they typically only do so related to the high court’s budget request.

Josh Blackman, a law professor at the South Texas College of Law at Houston, said Congress could pursue further answers from Thomas but his testimony would be an extraordinary step.

“Of course, you know, politicians want the guy under oath so they can try to pin him down on something and embarrass him, right?” Blackman said. “Which is really the purpose. I can’t see a world in which Thomas voluntarily testify under oath before the Senate.”

It would be difficult to force Thomas to testify, Blackman said, because the judiciary has a constitutional interest in keeping separate from Congress. Additionally, Congress’ main tool to compel testimony is a subpoena, which requires the courts to enforce.

Issuing a subpoena to Thomas would likely end up at the Supreme Court and “put his colleagues in a very weird situation,” by forcing them to judge their colleague’s case or recuse themselves.

Josh Chafetz, a law professor at Georgetown University Law Center who has studied legislative branch power under the Constitution, said there’s nothing that insulates justices from being called to testify.

Concerns about the independence of judges from other branches of government does not mean they are immune from subpoena, Chafetz said.

If a justice was allowed to duck a congressional subpoena, “the slogan of judicial independence ends up just being a slogan for ‘anything judges don’t want to do is judicial independence,’” Chafetz said.

The only congressional subpoena for a sitting Supreme Court justice was in the 1950s, Chafetz said. Back then, the House Un-American Activities Committee subpoenaed Justice Thomas Clark, who refused, and lawmakers didn’t pursue the matter.

Chafetz said that even if the courts do not enforce a subpoena against a sitting Supreme Court justice, Congress still has other avenues to seek enforcement. For example, it could use its inherent contempt power or cut funding for Supreme Court operations.

Long-term repercussions

Liberal group Common Cause sent a letter Tuesday to the Senate Judiciary Committee asking them have Thomas testify and “put it on the record for the American people,” as well as have Roberts testify.

“The U.S. Supreme Court has repeatedly proven itself incapable of policing itself without a code of ethics,” Common Cause co-president Marilyn Carpinteyro said in a news release. “It is time for Congress to hold hearings and pass legislation to establish a code of ethics for Supreme Court Justices to hold them to the same standards as every other federal judge in the nation.”

There could also be long-term repercussions if the Judiciary Committee turns the reporting on Thomas’ relationship with Crow into a fight over the justice’s testimony.

Robert L. Walker, a former chief counsel and staff director of the Senate and House ethics committees who now counsels on government ethics rules at Wiley Rein LLP, said the Senate would be treading on thin ice.

“To threaten or certainly to conduct an ethics investigation of a Supreme Court justice would raise separation-of-powers concerns and automatically raise concerns about the independence of the judiciary and of the Supreme Court,” Walker said.

Blackman said that if the Senate takes actions investigating Thomas or with the intent of establishing evidence to impeach him, it would create a “DEFCON one” standoff with the judiciary.

And Republicans could turn around and use that precedent to question Justice Ketanji Brown Jackson or other Democratic appointees to the court, as well as lower-court judges, he said.

“I don’t know where this leads, but the way the Senate, the way Congress works is that once a precedent is broken, then it is gone,” Blackman said.