This is all made much more complicated than it needs to be. Applying federal rules of procedure, civil or criminal, take your pick, there is almost no evidence which would be admissible in a court of law and what would be admissible is so ethereal that it could not sustain proof of the allegations by a preponderance of the evidence much less beyond a reasonable doubt.
The Shiffs and Nadlers of this world claim it doesn't matter, impeachment is a purely political device. That is a lie which should be obvious to anyone that can read. The Constitution mentions things like high crimes and misdemeanors trial, conviction, chief justice sitting as a judge, etc. Sounds like a legal proceeding to me. In fact impeachment is roundly held to be a quasi-legal proceeding.
So what? Being quasi-legal none of that court stuff matters. That's the big lie. Why do we have the Bill of Rights? Why have rules of procedure such as the hearsay rule, why those technicalities that allow criminals to escape prosecution? These laws express the American notion of fundamental fairness, freedom from being enthralled to a kingly government. That is a government that can roll over anybody like a tank for any reason or no reason at all. That was the genisis of the Bill of Rights. King Georgie III was doing all kinds of nasty things to suppress the colonists. Offend Georgie (or one of his minions) and go to jail and die. And the definition of fundamental fairness continues through new laws and Supreme Court cases such as Silverthorne Lumber which prohibits the admission of illegally obtained evidence. (fruit of the poisonous tree)
So while perhaps not directly applicable, nobody really knows, the fundamental rights which we see in court proceedings most certainly are relevant in impeachment proceedings as an expression of fundamental fairness as demanded by the American people.
So when evaluating evidence be mindful of what would be admissable on court. It's only fair.
The key elements in human thinking are not numbers but labels of fuzzy sets. -- L. Zadeh
