Andrew McCarthy: "The Constitution does not limit impeachment to incumbent officials. Article I endows the House of Representatives with the “sole Power of Impeachment” — i.e., the power to file articles of impeachment. It further empowers the Senate with “the sole Power to try all Impeachments.” Significantly, in prescribing the standard for conviction in the Senate, Article I, Section 3 states that “no Person shall be convicted without the Concurrence of two-thirds of the Members present” (emphasis added).

Note carefully: The Constitution does not say the impeached person must be a current officeholder. As we shall see, that makes perfect sense: The point of impeachment is to deny power to any person — not necessarily an incumbent official — whose high crimes and misdemeanors have demonstrated unfitness for a high public trust."

nationalreview.com/2016/09/impeach-hillary-clinton-congress-has-power-do-it/