From the Cato piece:
That actual crimes are not a prerequisite for impeachment is a settled point among constitutional scholars. Even those who take a restrictive view of the scope of high crimes and misdemeanors, such as Cass Sunstein, recognize that “an impeachable offense, to qualify as such, need not be a crime.”
160 Michael Gerhardt sums up the academic consensus: “The major disagreement is not over whether impeachable offenses should be strictly limited to indictable crimes, but rather over the range of nonindictable offenses on which an impeachment may be based.”
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