This In These Times article (posted to Alternet) claims the U.S. political blogosphere is bi-polar; the right is an extension of the Republican message machine, and the left is more a loose network of independents.

An excerpt:

The right blogosphere operates largely as part of the greater Republican message machine. Many of its bloggers are already part of that infrastructure, working as journalists for conservative publications, writing books and lecturing. Independent bloggers on the right hail from all walks of life, but the leading voices are either part of the political machine itself, like Mike Krempasky of RedState, or closely connected to the conservative media and think tank infrastructure, like Hugh Hewitt, Michelle Malkin and the PowerLine bloggers. The right blogosphere is a reflection of successful top-down Republican message control, and as such these bloggers are welcomed warmly into the fold.

As Garance Franke-Ruta writes in the April issue of The American Prospect, the right-wing blogosphere has also recently become useful to long-established political operatives such as Morton Blackwell, mentor to iconic GOP campaign strategists Karl Rove and Lee Atwater. In the recent Eason Jordan affair, the right blogosphere was credited with forcing the former chief news executive of CNN to resign over a controversial off-the-record comment. It turned out that many conservative blogs were part of this larger concerted effort. In the wake of this success, conservatives are now running what Franke-Ruta describes as "Internet Activist Schools, designed to teach conservatives how to engage in guerilla Internet activism," or what some people used to call "dirty tricks."

By contrast, the left blogosphere is populated by "citizen bloggers," who work in non-political occupations for a living and blog for reasons of personal interest. This sphere actually operates as a unique and potentially powerful political constituency rather than a part of the Democratic Party apparatus. Unlike their counterparts on the right, the lefty blogs have had to crash the party, but because they did it with energy, votes and money, they are making themselves a power in their own right.