James Madison, sometimes called the "Father" of our Constitution, wrote on October 15, 1788 (thirteen months into ratification), "The great desiderata on a Court of Impeachment are 1. Impartiality and 2. Respectability" (credibility).
Some Relevant Political Theory
Madison's mentor, 17th-C. philosopher Charles de Montesquieu, was not optimistic about the ability of "the people" of democracies to keep their civil officers from working for their own personal benefit rather than for the people's. He believed that monarchies and dictatorships are more solidly based. The love of titles can keep a king's civil officers loyal and the fear of death can do the same for a despot's. But a democracy is complex. Its final power, instead of residing in one king or one despot, resides in a collective known as "the people". de Montesquieu noted that the people of a democracy must rely on human "virtue" to keep their civil officers honest. He called this the "inherent weakness" of democracies.
Madison evidently hoped that an impartial, judiciary-based impeachment system, combined with another de Montesquieu principle (the separation of powers), could remedy this weakness. We'll never know whether Madison's hopes would have been realized had fellow-framer Hamilton continued his early support for the judicial trial of impeachments. As it happened, Hamilton changed his mind and successfully proposed the Senate to the Convention as the trier of impeachments. This occurred on the Convention's next-to-last day. The issue was important, as we have come to realize over the past two centuries. However, in 1787-88 no one issue, however important, was worth risking or delaying ratification of the proposed constitution. Despite this and other flaws, Madison worked with Hamilton and Jay to publish the Federalist papers in support of ratification.
US Politics Today
Our situation today is very different. In 1787 national disasters were almost certain to quickly follow any event that interfered with ratification. Today, a 3rd political presidential-impeachment trial, whether producing an acquittal or a conviction, would not threaten our existence. It might, however, establish us beyond redemption as a failed democracy.
It is true that there are more factors operating now than the assignment of a crucial judiciary power to a legislative body. This violation of the separation-of-powers principle is now combined with its own by-products and several independent co-incidents to make substantial impeachment amendments necessary. Two co-incidents that also require the transfer of the power to try impeachments to the judiciary necessary are the advent of the two-party system prior to the Civil War and the passage of the 17th Amendment in 1913. It is worth noting here that our impeachment difficulties and other governmental failures might have been avoided had we followed Jefferson's advice and scheduled constitutional conventions to be held at nineteen-year intervals.
Immediate Revisions Needed to the Senate Impeachment Rules
Some of the negative by-products of the mis-assignment can be remedied quickly by the Supreme Court under its judicial-review authority. They are:
(1) The exemption of the Senate and the House from the impeachment provisions by the Blount case in 1799. It was not clear whether he escaped because he was no longer a senator or because the senators claimed that they were not civil officers. In any case, it was contrary to constitutional intent.
(2) The unconstitutional Senate rule stripping the Chief Justice of the power to "preside" over presidential trials. The Chief Justices in the Johnson and Clinton trials failed to "preside". As a result, these trials were political rather than impartial.
(3) The failure to enforce the constitutional division between the House and the Senate impeachment powers. A number of senators in the Clinton trial publicly stated that they voted to acquit because they disagreed with the House decision that the charges were impeachable.
(4) The mistaken granting of due process to impeachment defendants. Hamilton makes it clear in Federalist 61 that impeachment law is a branch of law separate from criminal and civil law and that "due process" is inappropriate for impeachment cases. He wrote "This (impeachment-trial process) can never be tied down by such strict rules, either in the delineation of the offense by the prosecutors, or in the construction of it by the judges, as in common cases serve to limit the discretion of courts in favor of personal security..."