I know there are many here not fans of Mark Levin, but I respect hao legal analysis. His conclusion is that an impeachment trial after the President leaves office is not constitutional:
This is an area of the law where Constitutional Lawyers have differing opinions.
Step back and imagine Nixon but pretend he is a Democrat.
Articles of impeachment were approved by the House Judiciary Committee but he resigns before the Full Vote. (assume not pardoned)
He wants to run again but resigned just so there wouldn’t be a trial.
Should Congress be able to hold the trial.
The founders gave Congress a mechanism to deal with this.