The voting public has pretty reliably supported these pertinent declarations so far:
Trump lied
Trump obstructed
Trump committed crimes
The Mueller report does not clear him
These were not things that polled originally where they have for the last 6 months, in which a decent majority supports all of the claims. Even on impeachment, recent polls have us pretty close in favor of or against, single digits and sometimes margin of error. This seems like a very winnable public battle.
But even putting aside that, I think if the left is going to position itself as it has it the last few years as not angling for polling victories, moderation, big tent, whatever malarkey gets tossed out there, it's imperative to impeach. Trump has undoubtedly broken the law, the constitution, and has presented himself as entirely unfit. That this is true of many many many other presidents doesn't change that.
I can maybe, maybe buy that Trump is so perniciously bad that removing him from office is the most important goal, and that 2020 is the best chance to do that, and so, impeachment is a gamble. But that same argument then, again, leads us now to coalescing, at least right now, entirely behind
I'm not sure I buy that we can know for sure which option is 'the best' for electoral victory or ending Trump's presidency, but we do know that impeachment was built for this very kind of presidency. Nor do I personally want to vote for a serial hair sniffer and racist.
It also says a lot about the Democratic Party that if they choose to not impeach simply on the basis that the Republican senate will protect Trump (which they will), and that the body politic is so corrupt as to allow such behavior, than there is literally, truly, no reason for any Dem to not run on things from court packing to partitioning California to all kinds of increasingly un-institutional ideas. Since they won't do that, that leaves them as needing to at least take the institutional step of starting impeachment hearings.