Impeachment proceedings are not spelled out in the Constitution and in reality are political proceedings subject to the rules in the House and Senate set by their leaders. I doubt they are even subject to review by SCOTUS. What I just stated may get an early and ultimate test if Administration figures stiff the House on their subpoenas. It is most definitely NOT a criminal proceeding governed by criminal procedure rules. Ultimately, the proceedings are governed by what a majority in the House and 2/3 in the Senate vote to do.
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“You can vote your way into socialism, but you have to shoot your way out.” — Too fundamental to have an attribution