[Ed. – Another way to say it might be that a Senate trial can have no constitutional effect. I don’t know that it violates the Constitution for the Senate to hold a trial — the Senate can put on a performance of Twelfth Night if it wants to, or hold a monster truck rally — so much as that trying Trump after he’s left office can’t have the constitutional effect it would have if he were still in office.]
Jonathan Turley made a public recommendation earlier this week to President Trump and his legal team that I agree with — they should not participate in any Senate proceeding. Not participating would preserve a later challenge to the legitimacy of the exercise. He also covers a host of other unique problems that would infect such a proceeding, including whether the Chief Justice would participate in the trial of a private citizen which is not expressly provided for in the Constitution.
The purpose of an impeachment trial is to remove a public official from the office they hold. “Impeachment” by the House is the functional equivalent of an “indictment” — it is the issuance of a charge of misconduct against the seated official sufficient to justify removal from office. The “trial” in the Senate is the process for actually removing that official from office.
But what the Democrats are after is the collateral consequence of removal, which is to bar the individual from seeking the same office in the future.