Suppose I have (at least) 3 oranges and you have (at least) 5 apples, and I'd consider trading three of my oranges for four apples to be a net improvement to me, and you'd consider trading five of your apples for three oranges to be a net improvement to you. We have a situation, then, where we ought to be able to find a mutually beneficial trade, but there is a real danger that we get hung up on that fifth apple; even if our relative values are common knowledge — so that, among other things, we both know that we ought to do some kind of trade — it's quite possible we end up unable to agree to a deal, if we both dig in our heels, I insisting that you give me five apples for my oranges, and you insisting that I accept only four.
This looks like an economic model, and it is that, but it's also a model of politics, and while I think people have a tendency every four years to think that they're in the ugliest presidential race ever more from poor historical perspective than a monotonic decline in the state of American politics, it really does feel as though the "digging in our heels" bit has become worse in the last several years, especially (but not exclusively) on the right. I also think, though, that there's less common knowledge of values than people often believe, which creates more problems, the way that my insisting on six apples would in the opening parable.
I don't think it's inconsistent for me to add, though, that I also feel as though a common and vapid form of political discourse involves essentially decrying any attempt by the other side to negotiate terms of trade; in particular, when controversial riders get attached to popular legislation, especially but not only when those riders are topical, that seems entirely fair as part of negotiating a deal. I've noted elsewhere on this blog how I might change procedures such that these gambits would be less capable of blocking consensus bills, but certainly given the rules we have, something well between unilateral disarmament and complete obstinacy should be achievable.
in and of itself. Demanding huge concessions for passing a popular or urgent bill is antisocial, while adding something that one side mildly dislikes is very different.