I cant stand Trump, but I do think this impeachment affair is totally bogus.
Once the Russiagate affair fell to pieces the Democrats got desperate and are clutching at straws.
They have spent the last three years chasing Trump for bogus Russia collusion charges and now they are going after him over a phonecall that amounts to very little in my opinion.
It will back fire I believe.
The Mueller enquiry hasn't fallen to pieces. There have been 30+ indictments and 8 convictions of Trump associates. It was never going to lead directly to an indictment of Trump as that isn't possible but it did clearly state that Trump obstructed justice. And the evidence from that is part of one of the two articles of impeachment that are being proposed (the obstruction of Congress). The second one, abuse of power, is probably pretty clear if it was a regular court - which of course it isn't. And it's not just one phone call. There is also pretty good evidence of breaking the emoluments clause of the constitution but the Democrats have obviously balanced having too many issues cloud the PR battle.
They probably know the impeachment will not be confirmed in the Senate because of the 2/3rds requirement. But I think there are multiple reasons for going ahead;
(1) It gets republican Senators on the record which will impact re-election races next year.
(2) They won't convince very core Trump voters. But he actually needs more than that. His core vote is approx. 35% (slightly under). Depending on the breakdown in various states, Trump needs about 45% to get re-elected. So it is more focussed on the voters who swung from Obama to Trump in the last election
(3) Once the trial starts in the Senate, it is out of the Republicans hands. And has been seen in the committee hearings, once people are under oath and testifying, a lot more has come out than originally assumed. While the Republicans control the Senate votes, they won't control the proceedings (Chief Justice does). So it has the potential to cause further surprises
(4) I suspect part of the rationale is to lay down a marker. If there was no attempt to "call out" some of the activities, their concern is that Trump would get up to even more at the next election. In some ways, it may be more targeted at people who enable him who are not immune to prosecution.
I don't think they took the decision easily - in fact the debate about the futility of doing it and potentially giving a Trump a win caused them to hold off for a long time. But I think they eventually came to the conclusion that doing nothing would be morally wrong and lead to a normalisation of behaviour