Devin Gerald Nunes, chairman of the House Intelligence Committee, left, and Chief Justice John Roberts Jr.

The suggestion that Chief Justice John Roberts Jr. might be asked to testify before Congress in the wake of the controversial “Nunes memo” is drawing criticism and raising questions about the separation of powers.

During a radio interview with Hugh Hewitt on Wednesday, House Intelligence Committee chairman Devin Nunes (R-Calif.) said he and his staff were “grappling with” whether to ask Roberts to appear in his role as the appointer of judges for the Foreign Intelligence Surveillance Court.

The memo Nunes released on February 2 has been cited by Trump supporters as proof that the FBI misled the surveillance court in its 2016 application for a search warrant for former campaign advisor Carter Page. Though by law, Roberts appoints the judges for the court from among federal district court judges around the country, he does not have supervisory duties over the court.

“We have thought a lot about this. And the answer is we don't know the correct way to proceed because of the separation of powers issue,” Nunes said in the interview. “I'm not aware of any time where a judge has, for lack of a better term, testified before the Congress.”

In fact Supreme Court justices, including chief justices, have testified before Congress—but by invitation, not by subpoena, according to court historian Mel Urofsky. “Congress doesn't really have the power to subpoena a chief justice,” he said. “We have three coequal branches, and a subpoena like that would mean one branch has priority over the other.”

Rutgers University political scientist Ross Baker said that calling on a chief justice to testify concerning a specific case or controversy would be “more than inappropriate. It would be presumptuous.” Baker is author of Strangers on a Hill, a book about the relationship between Congress and the Supreme Court.

Until 2015, two justices would routinely appear annually before House or Senate appropriations committees to answer questions about the Supreme Court's budget. Since 2015, those discussions have taken place informally and behind closed doors.

In 2011, Justices Antonin Scalia and Stephen Breyer testified before the Senate Judiciary Committee, not about the court's budget but rather about the constitutional role of the judiciary. Senators were careful to emphasize that the justices had been invited not forced to testify.

“It is rare for sitting Supreme Court justices to appear” before Congress, Sen. Chuck Grassley (R-Iowa) said at the time. Sen. Patrick Leahy (D-Vermont) said that committee research found that Chief Justices William Howard Taft and Charles Evans Hughes had gone before Congress, as did Justice Robert Jackson.

Urofsky said Taft testified before Congress to push for passage of the Judiciary Act of 1925, also known as the “Judges' Bill,” to reduce the Supreme Court's workload. Hughes wrote letters to the Senate Judiciary Committee opposing President Franklin Delano Roosevelt's 1937 court-packing plan. Jackson scholar John Barrett of St. John's University School of Law said Jackson testified before Congress at least three times on issues including judicial ethics and his role as a Nuremberg prosecutor.

Separate from Nunes, House Judiciary Committee chair Bob Goodlatte (R-Virginia) last month sent a letter to senior judge Rosemary Collyer of Washington, the chief judge of the surveillance court, for more information related to the Page subpoena.

First revealed earlier this week, the letter drew criticism from Rep. Jerry Nadler, D-New York, who complained that Democrats had not been informed: “Had we been consulted, I would have advised the chairman that the committee does not ordinarily demand information from the judicial branch in this manner.”