You should go read this piece, which ends:

[T]he President isn’t really on trial here. His guilt is too obvious and established. The question here is whether the GOP, his party, will take any step to restrain an obviously bad acting President. The answer to that is pretty obvious at this point. But it’s that more than the President’s guilt that needs to be elucidated, proven. This may be a subtle distinction. But it’s an important one. Getting it right can, I believe, put right what seems intuitively out of balance in these proceedings.

Democrats are going through the motions of trying to convince their GOP colleagues, even if in practice they know this is not going to happen. They need to shift gears. This isn’t about convincing their Republican colleagues. It’s about showing them up, indicting them for the decision they have very clearly made. Frustration, outrage, befuddlement are all in their way expressions of weakness. Republicans are committed to defending President Trump at all costs no matter what he does. That’s not something to try change. It’s what you’re trying to demonstrate.