“The Senate shall have the sole Power to try all Impeachments. When sitting for that Purpose, they shall be on Oath or Affirmation. When the President of the United States is tried, the Chief Justice shall preside: And no Person shall be convicted without the Concurrence of two thirds of the Members present.
“Judgment in Cases of Impeachment shall not extend further than to removal from Office, and disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States: but the Party convicted shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law.”
Thank you for that quote. What that quote notably fails to mention and the language from Article I Section 2 granting the sole power of impeachment to the House also does not mention, is any limitation on who can be impeached. The Founders were very good at using the English language to express their intent. Had they specifically wanted to limit the power of impeachment to currently sitting office holders, I am quite confident they could have done so. Instead, the only language is that the sole power of impeachment belongs to the House. That would indicate that the House has the power to determine who can and cannot be impeached. In any case, Trump was impeached while still in office, so there’s really no Constitutional justification to say that the House lacked the power to impeach him.
Obviously, none of the above changes the fact that the House has twice impeached Trump, once for conducting diplomacy and once for giving a speech at a political rally. These impeachment’s are ridiculous on their face and the Senate should have simply voted straight away to acquit without the need for a trial in both cases. But that’s based on lack of merit, not on Constitutional grounds.