Democratic U.S. Rep. Adam Schiff of California said special counsel Robert Mueller should seek President Donald Trump’s sworn testimony Sunday, but seemed to acknowledge that outcome was unlikely.

Schiff, who chairs the House Intelligence Committee, said on NBC’s “Meet the Press” it would be a mistake for Mueller to conclude his investigation without interviewing Trump in person and under oath.

“I think the constraint that Bob Mueller is operating under is he had an acting attorney general who was appointed because he would be hostile to a subpoena on the president,” Schiff said. “And now he has a permanent attorney general who was chosen for the same hostility to his investigation and who would likely oppose that step.”

“I also think that the special counsel feels some time pressure to conclude his work,” Schiff added.

Schiff’s pessimistic assessment notwithstanding, Attorney General William Barr did not foreclose the possibility that Mueller could subpoena Trump during his January confirmation hearing for the attorney generalship.

“The question from me would be: ‘What’s the predicate?'” Barr told the Senate Judiciary Committee. “If there was a factual basis for doing it, and I couldn’t say that it violated established policies, then I wouldn’t interfere.” (RELATED: Clarence Thomas Clerks Dominate Trump’s Judicial Appointments)

The Supreme Court enforced a subpoena against former President Richard Nixon in the 1974 U.S. v. Nixon decision, ruling that he had to comply with special prosecutor Leon Jaworski’s request for the infamous White House tapes.

Whether the president must comply with a subpoena for testimony is a different question — one the high court has never answered. In one relevant historical incident, former Chief Justice John Marshall indicated that prosecutors could subpoena former President Thomas Jefferson to testify during the 1807 treason trial of former Vice President Aaron Burr. Jefferson never appeared and Marshall was speaking merely as the presiding judge, not for the Supreme Court.

Former President Bill Clinton testified from the White House by remote video feed before a federal grand jury in 1998 that independent counsel Ken Starr convened. A subpoena for Clinton’s appearance was revoked when Starr and the president’s lawyers reached a deal setting terms for his testimony.

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