When someone has been indicted the prosecutor must produce the witness and documentary evidence to prove the accused guilty. You can't indict someone and then at the trial tell the judge that you need to call more witnesses to prove that the person you already indicted is guilty. You have to prove your case based on the witnesses and evidence that you used as a basis to indict the person.  Read this again as it explains it so well. It is not the Senate's job to call witnesses. The House was supposed to send over -all- the evidence of guilt with the witnesses they chose to call. The House was now asking the Senate to do something that they aren't legally supposed to do, and it would have created a legal precedent for future impeachments if they had gone along with the House's games.