Bowman said the House prosecutors may try to push Roberts to rule on the admissibility of evidence. “If he’s forced to decide, I think he would make an honest effort to apply normal rules of judicial relevance,” Bowman said. “But I imagine he’ll try to avoid being put in that situation.”

In addition, Roberts might be able to cast tie-breaking votes — a role that’s typically played by the vice president, who can act as the presiding officer during regular Senate proceedings. The rules do not explicitly give the chief justice that authority, but Chase broke two ties when he presided over Johnson’s impeachment trial, so there is some precedent.

Roberts’ job will likely be more complicated than the one Rehnquist faced. Before Clinton’s trial began, the Senate’s majority and minority leaders — Republican Trent Lott of Mississippi and Democrat Tom Daschle of South Dakota — worked out the rules together, sticking closely to those adopted for Johnson’s trial. No such spirit of bipartisanship has guided preparation for President Donald Trump’s trial.