Does the Chief Justice Preside over an Ex-President's Impeachment Trial?

Conflicting signals from the Belknap impeachment

President Trump has been impeached, and the trial seems unlikely to begin until he is no longer the President. There is some dispute about whether the Senate can try an impeachment of an ex-official. But it has done so in the past, most famously in the case of ex-Secretary of War William Belknap, and for reasons adequately covered by others I think that is proper.

If it does, there comes a secondary procedural question. Who presides? According to Article I, Section 3, normally "The Vice President of the United States shall be President of the Senate," and in her absence, other Senators preside. But "When the President of the United States is tried, the Chief Justice shall preside." Should we think of the trial of ex-President Trump as the trial of a President?

I started reading through the Belknap Senate trial and quickly came across conflicting clues. It seems to me that the answer to this question depends on the precise theory under which ex-Presidents can be impeached, and both theories were mentioned during the Belknap trial.

One theory is this. Technically, there is no restriction on who may be impeached and tried by the Senate. Article II says that "The President, Vice President and all civil officers of the United States, shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors," but that provision technically establishes one of the consequences of impeachment -- removal -- it is not the impeachment power. The impeachment power is contained in Article I, and says "The House of Representatives … shall have the sole power of impeachment" and "The Senate shall have the sole power to try all impeachments." That is the impeachment power, with no limit on who can be impeached.

The other theory is this. Impeachments of ex-officers are permitted because impeachments of officers can't be defeated by the resignation or departure of the officer. Otherwise, officers could evade the Senate's judgment, and most importantly, could evade the Senate's power to impose disqualification from future offices, a power expressly mentioned in the Constitution. This theory might not extend to the impeachment of private citizens, etc., but it would allow impeachment of ex-officers as an extension of the power to impeach and disqualify officers.

I found people articulating both theories within the first few pages of the Belknap trial. Some speakers argued that the impeachment power had an unlimited domain of defendants, and that is why Belknap could be tried. Others argued that the trial "related back" to the original House inquest against Belknap, when he was an officer. (The idea of "relation back" may be familiar to modern litigators.)

On the first theory, the Chief Justice need not preside. An ex-President being tried is not the President, and the Senate's jurisdiction does not depend on his former official status. On the second theory, I think the Chief Justice should preside. If the premise of the Senate's jurisdiction is that it "relates back" to the original impeachment of the President qua President, then the President's procedural protections should be triggered.

However, I do not know which is the correct theory, nor do I know which theory the Senate will adopt if it proceeds with the trial. Perhaps this question is resolved later in parts of the Belknap trial I haven't gotten to. But it seems to me that figuring out the correct theory helps us figure out how the trial must proceed.

A final thought: I am not positive this works, but one way to avoid this question would be a theory that the Senate has the power to designate the Chief Justice as the presiding officer regardless of whether the President is being tried. Perhaps if the Senate and the Chief Justice both agree to it, the Chief Justice could preside by consent regardless of whether the Clause mandates it. That arrangement would allow the Senate to dodge this question.