That's the problem. The Democrats can't seem to win either way. They did try going in tough with their fight to stop Kavanaugh being confirmed. You saw how that worked out for them. Plus, all their attempts to fight back seem to do is further fuel the "See, both sides are equally bad" attitude among some out there.
I will say that I don't think their "we go high" thing is them hoping or thinking that the GOP itself will change their ways, though. I'm pretty sure they know full well that that party's return to sanity is a lost cause at this point.
But I do think they're hoping that if they present themselves as being above what the GOP is pulling, that'll encourage voters on the left, as well as any independent voters and whatever moderate conservatives may well remain, to support them simply on the basis of thinking, "Finally, a group of people that isn't a bunch of assholes." Because the fact remains that more people don't support this administration than do, and I think a large part of that has been because of how outright cruel and nasty it's been. So on that level, I can understand the whole "let's go high" attitude, to show how sharp a contrast there is.
Having said that, however, I do agree they can't run just on that. They should also figure out how to spread the message of their particular platform in a way that gets people eager to vote and support them, and I do fully agree they should make clear that they will fight back against Trump as much as possible, too, because yes, this administration absolutely needs to be held accountable for its shit. There are ways to fight back without having to stoop to their level of nastiness, after all, so they need to figure out how to best do that, in a way that can get voters on board.