In the immediately preceding post, I wrote,
Also, please remember that, whatever Orange Man and Moscow Mitch say about witnesses, the House prosecutors can still call on whatever witnesses they choose to call, the Chief Justice will have to rule on the request, and the full Senate will have to decide whether to accept the Chief Justice’s ruling or vote it down.
This is the plain meaning of the Senate’s “Rules of Procedure and Practice” currently in force for the conduct of impeachments. These rules explicitly provide that the chief justice “shall” preside over the trial, that it is the “Presiding Officer” who “shall direct all forms of the proceedings,” and that he may “rule on all questions of evidence including, but not limited to, questions of relevancy, materiality, and redundancy of evidence.”
Roberts’ powers are not unlimited. The rules explicitly grant “any member of the Senate” the right to object to Roberts’ evidentiary decisions, and if the chief justice stands firm in his opinion, a senator may demand “a vote of the Members of the Senate” on “any such question”—with a simple majority sufficient to overturn Roberts’ ruling. Nevertheless, the Senate’s authority is strictly constrained by the rules it has itself established. While it can reverse particular evidentiary rulings, it can’t bar anybody from appearing as a witness. Instead, it is up to the lawyers representing the House and the president to make these critical decisions, with the proviso that “witnesses shall be examined by one person on behalf of the party producing them, and then cross-examined by one person on the other side.”