I think the impeachment won't dent the Dems chances: that's a right-wing talking point that somehow scares even the mainstream.
Poor electioneering on the other hand, such as what cost Clinton 2016, that's a real possibility.
Well, for one thing, impeachment will rev up Trump's base and motivate them even more to come out and vote.
And it could also have an impact on poor electioneering... when the whole nation is riveted upon the impeachment inquiry and actually pays little attention to proposals and ideas for the nation from the Dem candidates. That's precisely why the GOP was commemorating a delay due to a ruling by a federal judge. They actually want to drag this on and on, because Trump thrives in controversy, and they can keep pushing the Witch Hunt angle.
I think that there are also chances that the inquiry will expose Trump so much that more independents and moderates will turn against him.
But I don't know what effect will predominate.
The bottom line is, I think that impeachment is a two-edged sword. We don't know to which side it will cut. One thing we do know: it won't remove Trump from office since the Senate won't convict, so, if there is a point to it, it's electoral (and yes, saying so is no shame - all impeachments are ultimately a political process, as lavishly demonstrated by the GOP impeaching a good and competent president who was leading a great economy, over lying about a blow job - it certainly distracted Clinton and made him less effective, despite his popularity, in helping Gore). So, what remains to be seen is if impeachment will be an electoral advantage for the Dems, or for the GOP. I hope it's the former but sometimes I'm afraid it's the latter.
And certainly, I'm not saying so because of anything the right-wing is saying. What they say only disgusts me; it doesn't influence me. It's out of my own concerns and attempts at analyzing the situation, that I fear the worst. I mean, at one point, polls showed a majority of Americans being against impeachment, and that's when Pelosi was frontally against it (while idiots like AOC were pushing for it). Now, the tide is turning and an almost absolute majority is for it. But this can still change, depending on who is most skilled with the narrative. If the GOP succeeds in pushing the version that it's a groundless witch hunt, it might actually hurt the Dems. If the Dems can expose Trump's abuse of power, then advantage Dems.