“The Senate is not in an advocacy role, but acts as a judge and jury,” Jipping said. “The House managers and [the] president’s lawyers will likely make motions procedurally to benefit them, and the Senate will vote on how it wants to proceed with the trial.”
After establishing a date, the Senate orders the House managers—members of the House acting as prosecutors—or their counsel to provide the Senate’s sergeant of arms with information about potential witnesses and other evidence to be subpoenaed for presentation during the trial.
“The Senate will determine questions of competency, relevancy, and materiality,” the Congressional Research Service report says. House managers, or prosecutors, are the first to make opening arguments and—if needed—examine witnesses. The president’s counsel then has the opportunity to cross-examine witnesses.
Stewart said he has serious doubts witnesses would be called in any Senate trial of Trump over the Ukraine controversy.
“At the end of the day, the goal of the House managers and president’s lawyers will be to sway public opinion to drive the vote,” Stewart said. The House managers and the president’s counsel may make motions or objections during the trial.
However, the presiding officer at the trial “may choose to put any such issues to a vote before the Senate,” the report by the Congressional Research Service says, and any senator “may request that a formal vote be taken on a particular question.” Short of a rules change—itself requiring a two-thirds vote—a trial of Trump would follow the precedent of the Johnson and Clinton trials, both held before the full Senate.
4 Keys to Understanding a Trump Impeachment Trial in the Senate