It says that the chief justice "shall preside" over the Senate trial of a president. Those who envision a powerful role for Roberts point to Senate rules that would allow him to make decisions on "all questions of evidence." In theory, at least, that might enable Roberts to rule on motions from the House managers seeking to compel testimony from White House aides, like former national security adviser John Bolton, and others who had been blocked by Trump from testifying.
Yet under Senate rules, it is the senators themselves who have the first and last word. They establish the procedures for the trial and can, by majority vote, overturn any of the chief justice's rulings. The extent of the chief's powerlessness was driven home to Chief Justice William Rehnquist at the beginning of President Bill Clinton's Senate impeachment trial in 1999. When Rehnquist asked Senate Sergeant at Arms James Ziglar how to turn on his microphone, Ziglar replied, "You don't. We control that."
"I don't know if 'shock' is the right word," Ziglar recalls, but Rehnquist certainly had "a sense of dismay" that even though he was going to be the presiding officer at the trial, "he really had absolutely no control of his courtroom."