Now we come to the heart of the standard for impeachment: "treason, bribery, and high crimes & misdemeanors." I'm going to skip over treason and bribery for now and move to the third test, certainly the broadest standard and one that is much less apparent than issues of treason and bribery. Black writes about the decision to use this phrase "high crimes and misdemeanors" at the Constitutional Convention:
Mason’s ready substitution of “high Crimes and Misdemeanors” indicates that he thought (and no voice was raised in doubt) that this new phrase would satisfactorily cover “many great and dangerous offences” not reached by the words “treason” and “bribery”; its coverage was understood to be broad.
Black, Charles L., Jr.. Impeachment (pp. 27-28). Yale University Press. Kindle Edition.But a wide-open and general term could run afoul of the abhorrence among the Founders about bills of attainder and ex post facto (after the fact) laws. In other words, making something illegal after it's done. The Constitution specifically prohibits bills of attainder and ex post facto laws. About this, Black writes:
When a congressman says, in effect, that Congress is entirely free to treat as impeachable any conduct it desires so to treat, he (or she) is giving a good textbook definition of a bill of attainder and an ex post facto law, rolled into one. Our Framers abhorred both these things, and we have never wavered from that abhorrence. It cannot be right for Congress to act toward the president as though these prohibitions did not exist. There may be no way to keep Congress from violating their letter or spirit, but the conscientious congressman has to feel them, in spirit at least, as bounding and confining the operation of the vague words, “high Crimes and Misdemeanors.
I say “in spirit,” because the letter of these clauses cannot always apply. As pointed out above, in connection with the question of criminal character of the impeachment proceeding, the words “high Crimes and Misdemeanors” are themselves too vague to satisfy constitutional standards of reasonably clear warning, in criminal statutes as applied in the ordinary courts; in this technical sense, the application of the quoted phrase to concrete cases must often be “ex post facto” in practical effect. But the spirit and equity of the bill of attainder and ex post facto clauses can to a large extent be followed if we treat as impeachable those offenses, and only those, that a reasonable man might anticipate would be thought abusive and wrong, without reference to partisan politics or differences of opinion on policy. The approximation of this result necessitates exploration of some further issues.
Id. pp. 29-30.
