[frogjerk]

He is not in office. He cannot be convicted of impeachment (removal of office). Anything after he is out of office would be a bill of attainder which is clearly unconstitutional and has been adjudicated many many times.

[Campion]

It has not been "adjudicated many times". There is, as far as I know, only one case where someone was impeached after leaving office, but they were not convicted. The courts never ruled one way or the other.

A "bill of attainder" is a law. This is effectively a judicial proceeding. To claim that something is a judicial proceeding if someone is in office but becomes a bill of attainder when they're out of office ... okay. It's an argument; whether it's a valid argument is something a court would have to rule on.