As Paulsen notes, this argument is “contestable.” But it nonetheless seems better than the opposite view, which would have the “almost absurd” consequences he describes. Michael Gerhardt, another leading expert on impeachment, similarly points out that “It certainly makes no sense for presidents who commit misconduct late in their terms, or perhaps not discovered until late in their terms, to be immune from the one process the Constitution allows for barring them from serving in any other federal office or from receiving any federal pensions.”
This view is also backed by historical precedent. In 1876, President Grant’s Secretary of War, William Belknap resigned to head off impeachment for corruption. But the House proceeded to impeach him anyway, and the Senate tried him. Ultimately, a majority of senators voted to convict, but not the necessary two-thirds supermajority.
Finally, the idea that former officials are subject to impeachment is backed by extensive original-meaning evidence, outlined by Brian Kalt in a thorough 2001 article that is probably the closest thing we have to an authoritativ