If you're unfamiliar with the general idea, DBx's "theory" in this context is Public Choice Theory, which essentially says that governments are composed of people and people have their own interests as well as those of the organisation in which they are embedded, which helps explain the many stupid decisions such as protectionism that all governments make. Acceptance of this theory, of course, leads you to conclude that government should be minimised1.
So if any of my dwindling band of left-wing (or of any political persuasion, but knowledgeable of politics) readers claim to know of any left-wing theory of government behaviour, do let me know in the comments.
Updates, since some of this was apparently unclear. The idea we're talking about is a theory of behaviour of the government, in the sense of, errm, how govt behaves. Not a theory of govt, in a sense like "where does legitimate govt come from?", or even directly "what should it's objectives be?", but in the sense of "how would you expect the people that compose a govt to actually behave, in real life?"
Also, I've realised the question itself is slightly "unfair", if viewed as a challenge (of course it you just view it as a genuinely meant question, which is what it was, then it isn't unfair). The public choice people do have a theory of govt behaviour, but as far as can be told they're pretty well the only people with a non-naive theory (other theories propounded in the comments are the std "working selflessly for the common good", which is obviously naive; and "Marx had some kind of theory", but that lacks detail). One might suggest that if you happen to have a theory of Thing X, then maybe you can score points by asking everyone else if they also have a theory of Thing X. But I don't think that's true in this case: how a govt will behave in actual practice is, when you think about it, too important not to have a theory for.
Is the theory itself actually partisan? Well, no. It's just a theory. Anyone could espouse it. But oddly enough the left, on the whole, doesn't, for the obvious reasons: the theory or it's consequence is skeptical of govt, and the left isn't.
1. [2019/01/04] On reflection, this sentence is far too crude (noticed while talking to CIP). Acceptance of the theory tends to push you in the direction of minimising govt. But it doesn't oblige you to accept that conclusion; you may have other ideas which lead you to think Big Govt is a good idea. But on the whole the sort of people who are prepared to accept PC are unlikely to have such ideas.
* Today we have naming of parts. I recall this from school. Though I think that omitted the motto.
* Hayek vs Hobbes and the theory of law.