In contrast, Chief Justice Rehnquist did his best to stay out of the fray during President Clinton’s 1999 trial and saw none of his procedural rulings challenged. He may have made his greatest impression with his robe, whose sleeves were decorated with gold stripes inspired by the Lord Chancellor’s costume he saw in a production of Gilbert and Sullivan’s “Iolanthe.”

Because trial procedures reflected Senate rules rather than those of the Supreme Court, “when most issues came up, he huddled with the parliamentarian, the person who actually knew this stuff,” says Frank Bowman, a University of Missouri law professor and author of “High Crimes and Misdemeanors: A History of Impeachment for the Age of Trump.”

Chief Justice Roberts is likely to follow the Rehnquist model, Prof. Bowman says. “I think mostly he’s going to sit up there and be a figurehead, every now and again maybe having to make some rulings on the scope of various arguments,” Prof. Bowman says. “He won’t want to get the institutional integrity of his own branch of government all tangled up with this highly partisan exercise.”

That may not be so easy in the age of social media.