On the surface, it might be taken as a liberal show, as its creators are certainly liberals and there are no recognizable conservatives the show. More importantly, if there were, they would appear as hopeless squares in that environment, which validates how liberals tend to think of conservatives in general. But it's more complicated than that.
In this sense, The Wire is the rarest and most precious of beasts: A work of art that's intensely political but rarely devolves into agitprop.
First of all, the show explodes any notion of being culturally simpatico to the modern black underclass, the staple of liberal inclinations on urban issues for decades. Clearly for those of us in the bourgeois world, those people are not us. The greatness of the show is that they are compelling in their own right.
I think there's an implicit contrast with The Sopranos. In general, the narrative of black people in America is much more prominent than Italians. But this doesn't necessarily extend to the criminal class, where The Godfather is a major touchstone of American culture. The consituents of Baltimore's drug wars are forgotten people. We want law enforcement to enforce some measure of quarantine on them away from bourgeois America, and as soon as that happens we won't spare them a second thought. Nonetheless, protagonists of The Wire are completely compelling anyway, while at the same time not making any excuses or rationalizations of their criminality. Through this prism we see that what happens to them is important, even if in real life it's not important to us.