I would think that all of his Tweeting is incriminating enough.
You're not alone. Near the end of the narrative, this comment on an ominous tweet by Trump: "Legal experts weighing in on Trump’s statements called his tweets blatant witness intimidation and suggested that the President could be guilty of obstruction (again)." Just a matter of time until he tap-dances on a land mine that blows up.
He’s subpoenaed to appear at a Republican-controlled Senate hearing rather than voluntarily appearing before a House committee. Obviously that’s going to be a politically tainted sideshow, probably trying to intimidate or entrap him. He’d be better off appearing at the House committee where he stands a chance of being heard.
If there are fireworks at the hearing, it might mean the GOP senators have had enough. Grab your popcorn!