The GOP defense of Trump has essentially cratered. "The Ukraine President felt no pressure" since he didn't know at the time of the phone call that the aid to Ukraine had already been withheld--and illegally to boot. The matter of Treaty-bound "investigation" was in the OP.
The withholding was illegal all by itself, and then matters go to when even hearsay is admissible, even in Court. The opening witnesses had been aware for some time that aid was being withheld, and unlawfully. The government employees assigned to Ukraine seem to have known about it. Ukraine officials seemed to know about it. Comments about it are admissible--usual federal rules of evidence.
Then if Ukraine President didn't know about the Banzai Surprise Attack already in progress: Then White House now has major explaining to do. At the moment, even hoping that the appropriations illegally not sent to Ukraine--might prompt some Russian response: Is then an appropriate inquiry. The Congress was going one way. State Department was Going a different way. Guliani was going nowhere, and Ukraine had badly equipped boots on the ground.
US Government provoking a Russian military initiative--met with a Vietnam-war like response?
"Crow, James Crow: Shaken, Not Stirred!"
(Being on the phone call, but not in the room(?): Too far away to be anything other than on the phone call--asking GOP?)