The choice of the Senate made sense for the Framers, who contemplated a republic without strong parties and a Senate whose members —elected by state legislatures until the Seventeenth Amendment was ratified in 1913—were expected to function in a less partisan, more deliberative, and wiser manner than their popularly elected counterparts in the House. The impeachment oath, which requires senators to “do impartial justice,” is not a quaint ritual to be performed with a wink and a nod, but a procedure required by the Constitution because the Framers intended the impeachment proceeding to be run much like a judicial trial.
Senators are thus constitutionally bound to follow what Justice White described as “a set of minimal procedures.” His opinion does not specify their exact contours, except to say that they must be the kinds of procedures a reasonable judge would regard as necessary components of a court trial. Because no reasonable judge would refuse to allow witnesses with personal knowledge of the facts to testify in an ordinary trial, it is the Constitution itself that establishes the right of House managers to call witnesses such as former National Security Adviser John Bolton and Acting White House Chief of Staff Mick Mulvaney. Both men are thought to have firsthand knowledge of the president’s purpose in holding up congressionally approved military assistance to Ukraine after a phone call in which Trump asked the country’s newly elected president, Volodymyr Zelensky, to investigate Joe Biden and his son Hunter.