> any issue one side takes up is immediately something that - if possible - is criticized and turned into a point of differentiation
This kind of "bothsame" response willfully overlooks that one party has gone to substantial lengths to offer compromises on bills, appointees, and policy goals, while the other very much hasn't.
You're totally right to point out that Republicans are much more shameless in this regard. I didn't intend to imply the two parties are the same here, but that's how it reads.
But even if the republican party+electorate are at "fault" here - I don't think that fact is going to help anyone, nor that Democrats are acting like little angels here anymore (if they ever did). They don't have the power to stop this dynamic, but they sure are participating pretty wholeheartedly in it now.
More fundamentally, the idea of open markets and in general an open society (i.e. the anti-Trump) was in some sense fallacious, and Democrats didn't admit this quick enough. A rising tide does not lift all boats - not automatically; at least. And there was precious little care for ensuring people had the negotiating position to actually profit from the changing times. And what rebalancing there was didn't actually serve to reduce inequality, merely to make it sting a little less - but people don't like being treated as inferiors needing a paternalistic pat on the head.
I think of it a bit like a prisoners dilemma: sure, it's utterly self-destructive to do what Trump (and by extension his voters) are doing. But it's partly a reaction borne of lack of other options: if they're going to get screwed, then screw everyone; time for a reboot. Only: that reboot isn't in sight, and all kinds of other nasty social habits are coming along for the ride.
I'm exaggerating a little as a point of debate, but you might say that the liberal elite (say) 15-30 years ago was being a little deceptive, and were doing so in a somewhat condescending, abrasive way, and that was plain dumb - because that helped cause something like Trump. For the playground analogy: maybe they started, but this fight was avoidable.
The idealogical polarization of the parties was really kicked into full gear by Gingrich. Trump's very clever at manipulating media to take this polarization to new heights, but the playbook it's Newt's playbook.
It certainly doesn't help the Democrats (and reasonable republicans?) that they appear weak when they try to be reasonable or compromise.
This kind of "bothsame" response willfully overlooks that one party has gone to substantial lengths to offer compromises on bills, appointees, and policy goals, while the other very much hasn't.