The Senate’s standing rules on impeachment give the chief justice discretion to “rule on all questions of evidence.” When Roberts rules, a majority of the Senate may overrule him. But if they don’t, his decision “shall stand as the judgment of the Senate.”
In ruling on the motion of a Democratic or Republican senator to subpoena a witness said to have relevant testimony and documents, the chief justice would have to reconcile two strands of his judicial philosophy in determining how “to do impartial justice.” On one hand, Roberts cares deeply about maintaining public faith in the judiciary’s integrity and fairness; and on the other he criticizes judicial activism (as in his “hands off” approach to state gerrymandering). Tellingly, however, in the chief justice’s 2019 year-end report on the judiciary, he called upon federal judges to “maintain the public’s trust that we are faithfully discharging our solemn obligation to equal justice under law.”
In an impeachment trial, the chief justice should give priority to maintaining the integrity of the system, even if this requires him to play a more active role than Chief Justice Rehnquist played in the Clinton impeachment trial.