If you want to argue that we can't convict Trump until we hear from Bolton, Mulvaney, Perry, and Rudy - I would completely buy that argument. Because the White House is preventing them from testifying, we have no choice but to rely on 2nd hand information.
If this was a court of law, the first-hand witnesses would be compelled to testify. If they won't testify during this phase, I do hope the Senate compels them to appear. The house is going to impeach specifically for obstructing congress by blocking testimony.
If I was a sitting US Senator - I would take the following approach:
- The first-hand witnesses need to testify. We should not remove a sitting president based on 2nd hand evidence when 1st hand testimony is available.
- If those witnesses fail to appear, then I would vote to convict on the Obstruction charge.
I think that's 100% reasonable. You do not remove a POTUS based on 2nd hand testimony and circumstantial evidence. At the same time, you protect a key aspect of the separation of powers. Any Senator that buys into Trump's claim of "absolute immunity" from Testimony by every single person in the Executive branch is truly undermining the constitution.
This is a very smart approach for middling Republicans to take and they could get middling Democrats to support that position.