The Constitution states, "When the President of the United States is impeached, the Chief Justice shall preside," rather than any senator or the vice president, who under the Constitution serves as the Senate president.
He said the framers' rationale for requiring the chief justice to preside in trials of a president stems from the natural conflict of interest of a presiding vice president.The framers believed the chief justice should preside, Amar said, "precisely because the vice president is next in line."But Bowman said even after a president has left office, a chief justice could still oversee a Senate trial."Any prudent organizer of the trial would insist that the chief preside," he said, adding that the presence of the chief justice of the United State would lend "legitimacy" to the proceeding.