Maybe I'm naive, but I think Democrats have gotten baited into the wrong set of questions regarding impeachment. The debate terms seem to be "Trump is awful, and it's imperative to remove him" on the one side, versus "Republicans control the Senate, and they'll never go for it" on the other.
It should go without saying that, in a functioning democratic system, these would not be the questions. On the one hand, impeachment is not a remedy for generically awful people, it's a remedy for high crimes and misdemeanors. It shouldn't be a tool for exacting political vengeance. On the other hand, precisely because impeachment should ideally be about rule of law, not political vengeance, it should be equally appalling the prejudged confidence by Republicans that of course they'd never impeach their own President. That's simply them closing ranks around a political compatriot -- it represents an obvious abdication of democratic duty.
So the right move, for Democrats, is not to promise impeachment. It's to be very earnest about impeachment. Impeachment is not about politics. It's about rule of law, wherever that takes us. The Mueller Report (among other sources) plausibly raises some very worrisome acts of misconduct by the President, which Congress should investigate. If that investigation leads to the discovery of an impeachable offense, then Congress should impeach. If it doesn't, then it shouldn't.
Any time a media figure tries to pivot the conversation back to "but Republicans in the Senate won't ever convict", be aghast -- not because they won't convict, but because they've prejudged the investigation. How could they say, in advance, that they won't convict the President unless they were admitting that partisan motives would take precedence over the outcome of the investigation?
The thing is -- this isn't just me being a starry-eyed idealist. This is a strategic thing, albeit strategic as a poor substitute for the ideal thing (where there was a chance in hell that Republicans cared about actual oversight).
If we learned anything from Benghazi and email-gate and all the rest, it's really that the outcome of the investigation doesn't matter. The constant, steady, drip-drip-drip of scandal is what matters. It helps if you've got something real to go on -- and in Trump's case, we clearly do -- and it really helps if it isn't seen as a mere political stunt (though, as Benghazi and email-gate also teach us, neither of those are really necessary either). Be earnest about impeachment -- not as a prejudged gambit in a political chess match, but as a procedural step in an investigative process. Then bleed the man dry.
From a strategic standpoint, it doesn't really matter whether the investigation ends in impeachment (let along conviction) or not. What matters is the cloud. And the longer it can be dragged out, the more consecutive days "Trump" and "corruption/obstruction/Russia" are in headlines next to each other, the better.