From the Summary: "The Constitution gives the United States Senate the responsibility for trying impeachments, but does not address the standard of proof that is to be used in such trials. This report concludes that an examination of the constitutional language, history, and the work of legal scholars provides no definitive answer to the question of what standard is to be applied. In the final analysis the question is one which historically has been answered by individual Senators guided by their own consciences."
Library of Congress. Congressional Research Service