Why does the Senate think they do not have to follow the Constitution?
Article II, Section 4:
The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.
They do not have the authority to impeach a citizen Trump. Only a President Trump...therefore Justice Roberts is who is Constitutionally required for that trial.