In short, the whole thing is about to blow up in the face of the Democrats, and they know it, which is why they keep complaining about how the Senate is handling the hearing
The impeachment of Donald J. Trump, when it comes to evidence and due process, reminds me of a scene from the Old West. "Yes, Sir, there is no evidence that you stole any horses, but we intend to prosecute you for resisting being hung for it."
Our country's founding was based on the idea of judicial fairness, and the self-evident truth that everyone is innocent until proven guilty.