I don't see what's so hard to understand, the democrats held an inquiry which included witnesses, right? They voted to impeach Trump based on the information and testimony they found, right? So, they passed their Articles of Impeachment to the Senate, which include everything they based they vote on, right? Then the Senate hears the arguments on both sides and asks their questions and get answers, right? After all that, if 51 or more of the senators don't think they have enough information, then they'll vote to call witnesses and the trial continues until the senators have heard enough. But if they already have enough information to determine whether or not to vote for removal, then there's no point in taking up any more time, and they'll have their vote for removal from office.
Look, the case that was voted on in the House is already supposed to be a complete case that warrants impeachment on it's own merits without further witnesses or information. Otherwise, they shouldn't have voted to impeach Trump until they did have a complete case with 'overwhelming evidence'. It's an impeachment after all, they voted to overturn the will of the people as expressed in the 2016 election, according to our laws. So IMHO, they shouldn't be bitching about more witnesses and more documents, I don't think the democrats have any standing to ***** if the Senate decides they've heard enough already. IMHO, I don't think in this case they'll need to hear from any new witnesses. And besides, they said it themselves, they can always impeach Trump a 2nd time if they get new information.