I think you hit it precisely. Let me presume to finish that sentence another way: The left has spent so much time defining the right that there is no longer any common ground on which to debate. Let me repeat that last, depressing clause: there is no longer any common ground on which to debate. And they intend to keep it that way.
You've seen it; we've all experienced it. Your opponent claims that he believes in freedom and it turns out that every single one of his applications represents a claim on your freedom, on behalf of society, of Saving The Earth, of the Greater Good. He believe in freedom, yes, but only the freedom to agree with him. She claims to believe in free speech, but only her own - yours to the contrary is offensive and "hate speech" and hence permissible to suppress. He believes that the Black Congressional Caucus is an expression of free association but any White Caucus would be intolerably racist. Don't bother trying to counter any of these arguments from principle; the rejoinder is always that you only disagree out of some sort of mental pathology or self-interest when it's really only a refusal to accept fantasy for reality.
I've made it very clear to my (nearly all) liberal relatives that politics is off the table for now for just that reason: we have no common ground. For some reason that seems to alarm them although I do not know why, exactly. Most of them haven't put much more thought into it than "hooray, my side won and those conservative meanies lost." And I don't argue politics with children.
The reason is: they know they are wrong.