The high court was considering Trump's argument that he is immune from prosecution for actions he took while president, but another issue is whether Smith and the Office of Special Counsel have the authority to bring charges at all.

"Did you, in this litigation, challenge the appointment of special counsel?" Thomas asked Trump attorney John Sauer on Thursday during a nearly three-hour session at the Supreme Court.

Sauer replied that Trump's attorneys had not raised that concern "directly" in the current Supreme Court case

"It points to a very important issue here because one of [the special counsel's] arguments is, of course, that we should have this presumption of regularity. That runs into the reality that we have here an extraordinary prosecutorial power being exercised by someone who was never nominated by the president or confirmed by the Senate at any time. So we agree with that position. We hadn't raised it yet in this case when this case went up on appeal," Sauer said.