Democrats response to who Republicans will call as witnesses is "OH YEAH! We'll call Mulvaney and Bolton and Pompeo". And what will the say, what's the worst they could say? That Yes, Trump wanted an investigation into possible corruption of Hunter and Joe Biden. After all if we dismiss the ridiculous holding up funds for 55 days something legally permissible, that's what is left, that the President asked a foreign country to investigate an American citizen that he may run against in the upcoming election and what Republicans are telling us is OK, even if that is true, it is not bribery and does not rise to the level of an impeachable offense. That is all that we could get out of all the people mentioned because that's what happened. I'm perfectly willing to admit that is what happened. I also know that Democrats were not willing to address the fact that Hillary did the very same thing by hiring Christopher Steele, a foreign national to go to a foreign country, Russia of all places and get something of value, information on Hillary's political opponent, Donald Trump. That is a fact. A fact that didn't bother the Democrats one bit.
Trump concerned about a new guy elected in Ukraine, one of the most corrupt countries on earth and wondering if this new guy will be any different than the old guy. How could we possibly know until he does something other than mouth the words? Well, he does something by putting together what looks to be a less corrupt Parliament that passes a new law that says Parliamentarians may be held legally accountable for corrupt acts, a law that did not exist in the old government. So after 55 days of the current aid being held up so Trump could be convinced that this new President is indeed different than the old President, he release the funds. And that's an impeachable act? It would be a dereliction of duty under the treaty we had with Ukraine that says we must be convinced that their corruption problem has been solved before we give them any funds. Was the possible corruption of Hunter and Joe Biden investigated and proven one way or the other? No. To not do so would be to say "this possible corruption, we won't bother to look at because Joe is running against Trump". Really? If you are running for President you are immune from investigation? Sure Trump could have gone about it in a more diplomatic way by asking our FBI and CIA to look into the matter but would that have made a bit of difference to Democrats? It's still Trump asking for an investigation of the Bidens because who ever Trump asked to investigate, they would in turn ask questions of Ukrainians, would they not? So Trump took the less judicious route.
My go to scholar on Constitutional matters is Johnathon Turley. Here's what he had to say.
https://thehill.com/opinion/judiciar...mp-impeachment