I'm not yet convinced that this impeachment endeavor will progress to the US Senate.
Prior to that, it must pass through the House by a 2/3 majority.
The sweet spot of effectiveness of impeachment as a political tool for the Democratic Party lies somewhere between NO effort to impeach, and Full Senate Trial to remove Trump - IMO. In other words, having the Trial to Remove, and being unsuccessful in such is WORSE than not having anything happen in the first place, because it serves as a indicator of pettiness, and impotency at even that.
In the absence of any real power to remove Trump, the utility of impeachment hearings is to keep allusions and innuendo of wrongdoing freshly in the MSM, on a continual basis. But perhaps this is also subject to laws of diminishing returns, insofar as the more fluff and nothing folks hear, the more weary they are of the story...and the more frustrated they are with lack of meaningful political action on their behalf...
Until/unless they have the numbers, why would the Democrats want the vote?