This section is from the book "The Constitutional Law Of The United States", by Westel Woodbury Willoughby. Also available from Amazon: Constitutional Law.
By Story it was held that, to be impeachable, the accused must be at the time in office. He says: "If, then, there must be a judgment of removal from office, it would seem to follow that the Constitution contemplated that the party was still in office at the time of impeachment. If he was not his offense was still liable to be tried and punished in the ordinary tribunals of justice. And it might be argued, with some force, that it would be a vain exercise of authority to try a delinquent for an impeachable offense, when the most important object for which the remedy was given was no longer necessary or attainable." 3 This view however, has not been accepted, and its reasoning would not seem to be adequate to support it. For, in the first place, it is recognized by the Constitution that the object of impeachment may be not only the removal of the accused from office, but also his disqualification to hold office in the future. In the second place, as will presently appear, impeachment may be based upon other than criminal offenses, and, therefore, the argument that the accused may be punished in the ordinary courts of justice has, in those cases, no validity.
3 Commentaries, § 804.
When articles of impeachment were brought against Senator Blount his counsel urged, inter alia, that the Senate having already expelled him from that body, he was no longer subject to impeachment. It was not urged, however, that this non-amenability to impeachment would have followed from voluntary resignation. "I shall certainly never contend," declared Mr. Ingersoll, one of his counsel, "that an officer may first commit an offense and afterwards avoid punishment by resigning his office." 4 Inasmuch as the Senate held that a Senator was not, under any circumstance, subject to impeachment, it was not necessary to pass upon the plea based upon his prior expulsion. The impeachment finally failed, not, however, upon the question of guilt, but upon the ground that the Senate was without jurisdiction for the reason that members of Congress are not civil "officers" of the United States.
In the case of the impeachment of Secretary of War Belknap, however, the issue was squarely raised whether a civil officer, in anticipation of impeachment, may escape by resignation from liability to trial by the Senate. By a vote of thirty-seven to twenty-nine, seven not voting, it was held that the jurisdiction of that body had not been ousted by the resignation, and by a later vote it was held that for this decision a two-thirds approving majority was not needed. And it may be noted that, in general, it has been held that the constitutional requirement as to the majority needed for conviction applies only to the final votes upon the question of guilt.