I thought of something. Regarding the question of the constitutionality of impeaching and convicting an ex President, it seemed rather clear to me. There are two penalties provided for impeachment, if convicted.. One is removal from office, which clearly cannot be done if someone is already out of office. The other is against future holding of office. That one, just as clearly, can be applied to an ex-President. So, I thought, the question of constitutionality is obvious. Of course it is ok to try the ex-President, so that one can apply the penalty of prohibition of holding Presidential office in the future.
Then I thought of something.
Can you impeach someone who has never been President, and is not currently serving in office? Suppose the President-elect does something which is very bad, and clearly impeachable, and makes it obvious that he is unfit for office, but which is not actually criminal. Could he be impeached? Impeachment is the only way a person can be prevented from taking office in the future, so if the person is to be prevented from taking office, that's the only way to do it.
One possible answer is that impeachment can only be applied to conduct in office, but I don't think that's a reasonable answer. Suppose it was discovered that a candidate was behind the assassination of a political rival before he won the presidency. The President can't be indicted, but he clearly ought to be thrown out of office (and then indicted), so he has to be impeached for things he did before taking office. So, can he be impeached before taking office?
I think the answer is probably yes, but I'm not so certain, and I'm not certain it's really a good idea.
ETA: This thread might move pretty fast as people comment on the live action from the Senate, so I apologize in advance if I miss a reply.
The answer is very probably, no. But technically it could be possible. It's complicated because the Constitution isn't very clear on what impeachment is intended to do or how it works. It doesn't say specifically who can be impeached.
The Constitution give the House the power of impeachment. It gives the Senate the power to try impeachments. That's it. The House technically could impeach anyone. And the Senate could convict anyone.
The Constitution says that "The President, Vice President and all civil Officers of the United States" shall be removed from office on conviction of "Treason, Bribery, or other high Crimes and Misdemeanors." It also says that the judgments by the Senate are limited to "removal from Office, and disqualification to hold and enjoy any Office".
Could the President be impeached and convicted of something other than treason, bribery, or other high crimes and misdemeanors and be removed from office? The Constitution says the President
would have to be removed if convicted for those things, but if convicted for other things the Senate
could decide to remove the President from office. Or maybe not remove him from office but bar him from holding future office (although that would be rather strange).
Similarly, could someone other than the President, Vice President, or civil officers of the U.S. be convicted and barred from future office? The Constitution does actually say that can't happen.
The widely held opinion is that impeachment only applies to the President, Vice President, and civil officers of the U.S. But there is some interpretation there. Impeachment really relates to action by people in public positions against the United States. But it also includes an inability to carry out the duties of the office or a lack of confidence for the person to hold a position of trust.
In 2010, Judge G. Thomas Porteous Jr. was impeached and convicted for corruption actions he took before he took federal office. But he was also convicted of providing false information during his confirmation. And he was also convicted on action he took after getting into office.
If a Presidential candidate assassinated his opponent, I think Congress would have to wait until he is in office to impeach. That's a bit silly, but obviously other actions could be taken in the meantime. The administration would not work with the President-elect and instead work with the VP-elect. Or if the VP-elect was in on it, work with neither and work with the House Speaker to become acting President until a new election is held.
If in 2016 Trump had done exactly what he did in 2020, Congress could not have barred him from holding office in the future because he was not holding office at the time. Should the exact same fact patterns result in a bar from holding future office simply by virtue of whether the person so happened to be holding office at the time?
There are some complicated issues. There are many other questions that could be raised. I don't think Trump's defense really dug their heels into the tough questions of Constitutionality.