I read an article from some retired federal judge who said the constitution states impeachment is for an incumbent only and a trial would be unconstitutional
A judge should know better. https://constitutioncenter.org/interactive-constitution/full-text
Impeachment is done by the House and it is covered in Article 1 Sec. 2. Nowhere does it state it is for incumbents only. This is all it says.
The House of Representatives shall chuse their Speaker and other Officers;and shall have the sole Power of Impeachment.
A trial in the Senate is covered in Article 1 Sec. 3. I think unfortunately the SCOTUS would rule it is ok as one of the punishments is to bar the person from holding further office.
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.
Don't forget the entrenched politicians want Trump gone forever.
The part I highlighted, this is how the R's will stab him in the back if they want to, while pretending to defend him.
Right now the D's have 48 seats with 2 Independents who will vote with them.
If the R's make a big stink about this being unfair/unconstitutional and they will boycott the proceedings, all they have to do is get enough to sit out so that the D's votes encompass the 2/3rds majority.
If they start talking boycott, this is their plan and people need to light up their phone lines and let them know, we know what you are doing you back stabbing bastids.