There's been a lot of discussion about the upcoming Senate trial of former President Donald Trump following his second impeachment. The topics of discussion include whether he actually committed an impeachable offense to the impeachment procedures in the House to whether it's even constitutional to hold a post-impeachment Senate trial for a former president. Most of the people who weigh in on social media against proceeding usually throw around the assertion that the whole process is unconstitutional while unable to point to the part of the Constitution that is being violated.
Let's start with the Impeachment itself. While it might seem counterintuitive, an impeachable offense, within certain bounds, is what the House of Representatives says it is. The term "high crimes and misdemeanors" is not defined in the Constitution itself, but it's meaning at the time the Constitution was written suggests that it was interpreted broadly and not meant to be tied to any specific laws or ordinances. While "incitement of insurrection" may not be provable in a court of law, it does not need to be. While the recent impeachment may not conform to legal proceedings we have become used to after watching decades of crime procedurals and cop shows, if a majority of the House of Representatives decides that an action by the president is impeachable, then he's impeached. It may not seem right or fair, it might seem to be partisan head hunting, but it does not contravene the Constitution.
A specific objection that I saw today was that the recent impeachment was unconstitutional because Trump's words that have been construed as incitement to insurrection were protected speech under the First Amendment. This objection ignores the fact that not all speech is protected speech, the cliché "you can't yell 'fire' in a crowded movie theater" is illustrative of this point. One might reasonably argue that Trumps' words didn't constitute an incitement to insurrection, but you can't argue that it was automatically protected speech just because it was speech. Even First Amendment protection from government censorship does not immunize one from the consequences of that speech.
An objection to proceeding with the Senate trial is that Trump cannot be removed from office as a result of a Senate conviction because he is no longer an occupant of the office. There are two ways that this is argued, one is the practical, the other constitutional. The practical objection is that it's a waste of time, since he's already removed by dint of his term having expired n January 20th. The answer to this objection is that upon conviction Trump can be subject to "disqualification to hold and enjoy any Office of honor, Trust or Profit under the United States". So it's not really a moot point, since there are further consequences beyond removal, which is moot. The Constitutional objection is a bit messier, and not totally clear.
One view is that the impeachment clause of the Constitution lists those people who are subject to impeachment. They are the President, Vice President, and all civil officers of the United States (which includes federal judges and cabinet officers) and that an ex-president is not on that list. The contrary view is that nothing in the Constitution prohibits a former president from being impeached or being tried in the Senate after impeachment. There is only one precedent upon which we can refer to: in 1876 Secretary of War William Belknap resigned as the House of Representatives was contemplating impeachment. The House impeached him anyway, and the Senate held a trial, with a majority agreeing that it retained impeachment trial jurisdiction despite the resignation. He was acquitted, with a majority that was less than the two-thirds, voting to convict. A number of Senators that voted to acquit expressed doubt about whether an impeachment of a former official conformed with the Constitution. This precedent has never been applied in the case of a former president.
So there's nothing that unambiguously permits or prohibits the impeachment or subsequent Senate trial of a former president, despite assertions of unconstitutionality by the Trumpists. Whatever the scholars or those with unwarranted confidence in their knowledge of the Constitution may believe, the trial has been scheduled.