As I was writing a lecture on the impeachment of Andrew Johnson and thinking of those in favor of a broadly defined constitutional approach to impeachment, I came across a succinct statement authored by legal scholar John Norton Pomeroy in 1868 (he published it in 1870). Pomeroy and others were not so terribly concerned that an executive or other officer might act illegally, but rather that they might abuse their powers. I welcome any and all comments.
The importance of the impeaching power consists, not in its effects upon subordinate ministerial officers, but in the check which it places upon the President and the judges. They must be clothed with ample discretion; the danger to be apprehended is from an abuse of this discretion. But at this very point where the danger exists, and where the protection should be certain, the President and the judiciary are beyond the reach of Congressional legislation. Congress cannot, by any laws penal or otherwise, interfere with the exercise of a discretion conferred by the Constitution...If the offense for which the proceeding may be instituted must be made indictable by statute, impeachment thus becomes absolutely nugatory against those officers in those cases where it is most needed as a restraint upon violations of public duty.