It's a special era of lawlessness when the criminals get to decide whether they will come to the trial or deign to testify.

TrumPutin huddles behind legal protestations of innocence like "the call was perfect, the call was wonderful, the call didn't happen the way the transcript said, the call never happened, the call was made by Obama, I just got a voicemail."

And the others - ambassadors, personal attorney, secretary of State (or what is left of State), Attorney General (leader of what remains of the Justice department) - all claim executive privilege, inconvenience, the claims of the Founding Fathers, or amnesia.

TrumPutin makes it difficult when he denies that he asked a foreign government to investigate his political rival for him, then claims omnipotence to ask a foreign government to investigate his political rival for him, then makes a public appeal to a foreign government to investigate his political rival for him, then denies he asked a foreign government to investigate a political rival for him. 

The question is whether to impeach the scoundrel, or to institutionalize the lunatic. 

Alas, his Cabinet, personal attorney, ambassadors, advisors, and Vice-TrumPutin can't be impeached or institutionalized with him.