John Sides and Eric Lawrence write about their research into who reads political blogs and why:

How might political blogs and their readers affect the presidential campaign?

They will not change many voters' minds because the vast majority of their readers are already members of the choir and hold strong opinions about politics. So don't expect political blogs to make Democrats vote for John McCain or Republicans embrace Barack Obama. If political blogs change opinions, they will more likely do so indirectly -- by uncovering new information that is then amplified and discussed in media that reach a broader, and less partisan, cross section of the public.

This is all true enough but also, I think, an unduly limited way of looking at things. For one thing, having a relatively unpersuadable audience is, I believe, common to all explicitly political media. Only people who like following politics would tune in to Meet The Press and people who like following politics usually have strong views about politics and are thus unlikely to be swayed by things they watch there. But there are more questions to be answered than "should I vote for the Democrat or the Republican in November?" Blogs are much more likely to persuade people on issues like "John Edwards or Barack Obama" or even more so "as someone who doesn't even live in Maryland, should I care about the Al Wynn versus Donna Edwards primary?" or "is the telecom immunity provision of the proposed changes to FISA a big deal?"

Blogs are a niche medium for political obsessives, so they tend to impact readers' opinions on questions that normal people just wouldn't bother having opinions about at all. That's not the same as saying that no persuasion happens and it's all preaching to the choir. It's more like the members of the choir talking about choir-related issues that others may not really care about.

Photo by Matt Stoller used under a Creative Commons license