One thing that Schiff and his colleagues made abundantly clear is that they do not have confidence in the articles of impeachment for which they voted last month. If they did, they would not be insisting upon testimony and the release of further documents and saying things like “there’s a good way to find out what happened on that call” in reference to conversations between the president and his lawyers. The explicit premise of Schiff’s appearance in the upper chamber is that he and his fellow House Democrats know exactly what Trump said and why. At this point, on the basis of evidence already available, the case ought to have been made to their own satisfaction. If they had really thought it necessary to secure further documents and testimony, they should have taken the matter to federal court. Given the absence of material that they now claim is essential to proving their allegations (despite their not, of course, having seen it), it is difficult to see why they bothered voting for impeachment in the first place.
Or is it? At the beginning of December it had become obvious that the impeachment process was taking too long. If it were going to be wrapped up within anything like a reasonable timeframe, it would require an immediate vote in the House, followed by a swift trial in the Senate. The former was, as one might expect, forthcoming; the latter was not a question Democrats would be in a position to decide. Perhaps a case for impeachment tighter than the present one could have been made, but this might have required dragging things on for several months, as courts adjudicated their requests for testimony from various officials. Instead, the House gave it the old college try, and voted on the basis of what was available.