A crucial issue--or what should be a crucial issue--in any Senate trial of impeachment should be the appropriate standard of proof by which to judge the evidence, as it is in any trial. The constitutional text about impeachment does not address the standard of proof, so sound judgment and legal reasoning must apply here. Black, writing in 1974, splits the baby as it were, suggesting that something more than a civil trial stanard of a preponderance of the evidence (more likely than not) but less than proof "beyond a reasonable doubt" (the criminal standard) should apply. I think that this take makes the most sense. Black writes:
[F]or example: “Did the official on trial perform an action because a campaign contribution was given, or was this a coincidence?” Either is possible; the “finding” of this “fact”—the “fact” of corrupt or blameless motivation—may entail the drawing of complicated inferences from circumstances; the circumstances themselves may be proven with more or less clarity. Before the “fact” can be “found,” the trier of fact must decide on a standard of proof. Should he find an impeached president guilty of corruption if it seems slightly more likely than not that a corrupt motive was present? Or should a finding of guilt have to rest, as in a criminal trial, on evidence which leaves no “reasonable doubt”? It makes a big difference, as the example I have just given shows.
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What is the right standard for judging guilt in an impeachment proceeding? Of course we don’t know the answer with any sureness; we have to work it out for ourselves. As with so many constitutional questions, we have to ask what is reasonable, and the reply is here far from obvious. Removal by conviction on impeachment is a stunning penalty, the ruin of a life. Even more important, it unseats the person the people have deliberately chosen for the office. The adoption of a lenient standard of proof could mean that this punishment, and this frustration of popular will, could occur even though substantial doubt of guilt remained. On the other hand, the high “criminal” standard of proof could mean, in practice, that a man could remain president whom every member of the Senate believed to be guilty of corruption, just because his guilt was not shown “beyond a reasonable doubt.” Neither result is good; law is often like that.
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Weighing the factors, I would be sure that one ought not to be satisfied, or anything near satisfied, with the mere “preponderance” of an ordinary civil trial, but perhaps must be satisfied with something a little less than the “beyond reasonable doubt” standard of the ordinary criminal trial, in the full literal meaning of that standard. “Overwhelming preponderance of the evidence” comes perhaps as close as present legal language can to denoting the desired standard.
Black, Charles L., Jr. & Bobbitt, Philip, Impeachment: A Handbook, New Edition (p. 16-17). Yale University Press. Kindle Edition.
