Game over.
Partisan HackThey could say he's compromised because of his Ukraine background, that he'a partisan, but he will make a strong witness. The idea that this isn't some partisan "scam" or in the presidents words "lynching" will have to be doubled down on again and again, by officials like this, who are largely considered beyond reproach, which of course, the Republicans will do anyway.
Keep the evidence coming.
Game over.