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Thesis: In the US, if impeachment proceedings begin against an officer, and he resigns, they may continue. If they begin, and his term runs out naturally, they may not.
Could impeachment proceedings start against me, Eric Rasmusen? I am not an officer of the United States, though I used to be one, I think. I was a GS-9, if memory serves me right, as a 21-year-old intern with the title Economist in summer 1980 in the Hyattsville "Center for Health Services Research". I wasn't appointed by the President or confirmed by the Senate, though, so I don't know if that counts as an "officer". But one might argue that though I can no longer be removed from office, impeachment matters because if convicted I would be ineligible for future appointment or election as, say, Chairman of the Council of Economic Advisors, and while I remain eligible I'm a threat to the nation. That argument is fallacious.
A second case to consider is whether impeachment proceedings could be begun against Barack Obama, a former President. I think not. He is not an officer of the United States. People call former Presidents by their title, as an honorific, but that is a horrible and undemocratic custom, entirely contrary to the idea that all citizens are equal and when the President steps down from office he returns to being just another citizen.
The only interesting case is that of a former officer of the United States whose impeachment proceedings began while he was an officer, but did not finish while he was an officer. That is the situation with Donald Trump, former President Trump, now Mr. Trump.
We should distinguish, though, between the case of a President who resigns in the middle of an impeachment and case of a President whose term ends naturally. I suggest that impeachment proceedings can continue if the President resigns in the middle of them, but the proceedings expire if his regular term of office expires.
It is not altogether ridiculous that a President be impeached after he is no longer President. It is ridiculous to being proceedings when somebody is no longer an officer of the United States, to be sure. This is a difference between the United States and England, where impeachment is a criminal proceeding designed to punish someone the legislature cannot entrust to the justice of the King's courts, which are not independent. Thus, the example of Warren Hastings does not apply. But what if proceedings are begun when the person is an officer, but he resigns to avoid being convicted and losing his ability to serve in the future, when the political climate has changed? In such a case, it seems reasonable that we would not want his resignation to stop the proceedings. That was the case with Governor Pa Ferguson in Texas in the 1920's or Secretary of War Belknap's impeachment.
Trump's case is neither of these. He was impeached while President--twice-- but his term came to its natural end. Could the Senate then try him after his term expired? Note that if this is so, then the Senate could have a trial based on either impeachment--- the first, in 2020 on charges of corrupt dealings with the Ukraine, or the second, in 2021, on charges of inciting rebellion. This is not a criminal proceeding, so double jeopardy does not apply.
It is stretching the language of the Constitution to say that someone who is not President can be impeached, but there's a reasonableness argument for doing that stretching if the reason the person is no longer President is that he's trying to escape impeachment. There are actually legal doctrines to this effect. I get them confused unless I look carefully, but I think estoppel might apply. Also, it is actually questionable whether a President can legally resign, even though it's been done and although inferior officers can resign. The Constitution doesn't allow it, does it? ANd if you do something illegal, an equity maxim says you are not allowed to benefit from your own unlawful behavior in legal procedure, I think. The murderous heir rule is a statutory version of this (was it in common law too?) Behind all of this is the commonsense notion that you can't squirm out of punishment by manipulating the system unlesst he system's rules really allow you to. Some legal research could make this precise. But it differentiates Trump's case: he did not resign to interrupt impeachment proceedings.
A prudential argument for my position is that if we follow the practice of letting impeachments run past the natural term of office, we will get situations exactly like our present one, where to meet the deadline for "commencing proceedings", the House of Representatives skips the usual lengthy procedure of committee hearings, full discussion, and plenty of time to formulate and refine impeachment charges, and instead just throws together some flimsy pretext and rushes it to a partisan vote. Note that this is equally a bad thing whether (a) there is no good reason to impeach, or (b) there *is* a good reason to impeach, but it will get lost in the rush to throw something together, so the Senate will be voting on the wrong charges.
Another interesting consideration is that an ex-President does not have the White House legal counsel or the Justice Department to help defend himself. Or does he? It would be odd, since there is a new President who is their boss. The new President should really volunteer the resources for the old one, though.
In all of this, remember that when we're discussing impeachment after someone has left office, we only have to be concerned with offenses that are impeachable but not criminal. Criminal offenses can be left to the normal course of criminal justice. If an official stabs someone, he can be indicted, prosecuted, convicted, and imprisoned. If he has not resigned or been fired, however, he still holds his office, so he still needs to be impeached, uncontroversial though that vote might be. So we need both criminal law and impeachment for people still in office. Some people think that it is improper to impeach someone for an offense that is not a crime. That is completely wrong. The Constitution does not require that. And it is easy to imagine offenses that are impeachable but not criminal--- for example, the President ordering American troops to invade Canada because he wants to seize all the maple syrup there. That is not a crime, but surely everyone thinks it is beyond his powers and he should be impeached for it. Thus, we do not have to worry about a President committing crimes with impunity towards the end of his term because he thinks there won't be enough time to finish impeachment proceedings. There won't, but he can be tried and punished just like any other citizen. Note, too, that the purpose of impeachment in America (unlike the United Kingdom) is not to punish anyone. Rather, it is to protect the country from the offending officer, by removing him from office and preventing him from running again. That this result makes him feel bad, as punishments do, is incidental.
Pa Ferguson's Impeachment in Texas
Ferguson v. Maddox, the Pa Ferguson Texas case from the 1920's, and the Impeachment After Leaving Office article that discusses it. Ferguson resigned after his impeachment but before his Senate trial. He was impeached after he started a big fight with the President of the University of Texas. In 1924 he wanted to run again, but his wife, Ma Ferguson, ran in his place because the Texas Supreme Court ruled that his Senate trial was valid and disqualified him from office. She ran against the KKK's candidate and won.
Secretary of War Belknap's Impeachment
When Belknap's case was exhibited to the Senate by the Housemanagers, it was framed as articles of impeachment against "WilliamW. Belknap, late Secretary of War."' That first sentence wouldprove the entirety of the issue for a minority, but a determinativenumber, of senators. --Jonathan Turley, "Senate Trials and Factional Disputes: Impeachment As a MadisonianDevice," Duke Law Journal 49, no. 1 (October 1999): 1-146
Citations to Investigate
"Review: Impeachment, Attainder, and a True Constitutional Crisis: Lessons from the Strafford Trial," Craig S. Lerner, The University of Chicago Law Review, Autumn, 2002, Vol. 69, No. 4 (Autumn, 2002), pp. 2057-2101 .