Gary L. McDowell, "High Crimes and Misdemeanors": Recovering the Intentions of the Founders (1999) here:
[I]t was essential to their way of thinking to make clear that impeachment was a political process dealing with political wrongdoing and not a part of the criminal justice process. Thus, they made clear that punishment for impeachment could not "extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States." But they emphasized that impeachment was not a bar to prosecuting criminal acts that the person impeached may have committed by noting that "the Party convicted [of impeachment] shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law." Thus, although an indictable crime may be deemed an impeachable offense, impeachable offenses are not simply limited to indictable crimes. ...
In all of the English cases the political nature of the offenses charged in impeachments was revealed by the use of the word "high" to modify both "crimes" and "misdemeanors." The use of "high" in "high crimes and misdemeanors" did not refer to the substantive nature of the offense, that it was a particularly serious offense, but that it was a "crime or misdemeanor" carried out against the commonwealth itself. This use of "high" to distinguish crimes and misdemeanors against the society as a whole derived from its use in distinguishing "high" treason from "petit" treason. Alexander Hamilton summarized this understanding of "high Crimes and Misdemeanors" as adopted by the Federal Convention in his explanation of the impeachment process created by the Constitution. The objects of impeachment, he noted, "are those offenses which proceed from the misconduct of public men, or, in other words, from the abuse or violation of some public trust. They are of a nature which may with peculiar propriety be denominated POLITICAL, as they relate chiefly to injuries done immediately to the society itself."
In all of the English cases the political nature of the offenses charged in impeachments was revealed by the use of the word "high" to modify both "crimes" and "misdemeanors." The use of "high" in "high crimes and misdemeanors" did not refer to the substantive nature of the offense, that it was a particularly serious offense, but that it was a "crime or misdemeanor" carried out against the commonwealth itself. This use of "high" to distinguish crimes and misdemeanors against the society as a whole derived from its use in distinguishing "high" treason from "petit" treason. Alexander Hamilton summarized this understanding of "high Crimes and Misdemeanors" as adopted by the Federal Convention in his explanation of the impeachment process created by the Constitution. The objects of impeachment, he noted, "are those offenses which proceed from the misconduct of public men, or, in other words, from the abuse or violation of some public trust. They are of a nature which may with peculiar propriety be denominated POLITICAL, as they relate chiefly to injuries done immediately to the society itself."
