During the controversy over CBS’s National Guard memos last September, Jonathan Klein, a former CBS News executive who’s now the president of CNN, took to the airwaves to the describe the average blogger as “a guy sitting in his living room in his pajamas writing.”

That image never quite fit Joshua Micah Marshall, who runs the influential left-leaning blog Talking Points Memo, and, before his recent move to Manhattan, did much of his blogging from his local Starbucks in the Dupont Circle neighborhood of Washington D.C. – clad in more than pajamas, presumably.

In fact, for a handful of political bloggers who’ve built large readerships, blogging as an enterprise has recently reached a tipping point of sorts – one that’s moving them ever further from the amateurs-in-pajamas model. Selling ad space has allowed a small number of well-read bloggers to generate enough income to support themselves, turning what was once a labor of love into a commercially viable business model.

Mr. Marshall has other sources of income from his conventional journalism – he’s written for the New York Times, the New Republic, and the American Prospect, among many others, and he writes a regular column for the Hill, a newspaper that covers Washington politics – but last year, Talking Points Memo “brought in enough for me to live on, put it that way,” he says, as his dog Simon nips at his heels.

And though he’s long been one of the few prominent bloggers whose site features original reporting – he frequently makes calls to congressional offices or government agencies and tells readers what he’s found out – Mr. Marshall has lately gone a step further.

Over the last month, he’s reacted to what he sees as a fraudulent attempt by the White House to phase out Social Security by turning his massive readership into an army of volunteer reporter/activists. In doing so, he’s made the site sometimes feel more like activism than journalism.

Other blogs, of course, have been hotbeds of political activism over the last year: Daily Kos and MyDD on the left, for instance, and RedState.org on the right. But perhaps none so far has brought to the cause Mr. Marshall’s credentials as a mainstream – albeit opinionated – journalist. In doing so, Mr. Marshall may have helped to reinvent the medium.

The pitch to potential advertisers is appealing. At the height of the election season last fall, he could promise them almost three and a half million visits – and almost 700,000 unique eyeballs – per month.

And not just any eyeballs. Fifty-four percent of Talking Points Memo readers earn over $75,000 a year, and 14% earn over $150,000. Some 85% have a college degree, and 83% have contributed to a political or nonprofit cause, making the site an attractive choice for candidates and political groups looking to raise money and recruit volunteers. Advertisers pay anywhere from $200 a week for a less prominently positioned spot, to $750 a week for the prime position, which appears right alongside Mr. Marshall’s most recent post. Paramount Pictures was a recent advertiser.

Mr. Marshall isn’t alone in figuring out how to make blogs pay. Blogads, a company that takes about a 20% cut for handling the technical and payment issues for blogs that sell ads, including Mr. Marshall’s, had over 500 clients last year. During last year’s election campaign, Daily Kos took in $20,000 a month by some estimates.

But it’s the writing that keeps the whole enterprise running. Lately, Talking Points Memo has been one-stop shopping for the complex legislative politics surrounding the Social Security issue. When the public debate started late last year, says Mr. Marshall, “it seemed like such an important thing to me, and it seemed like the people who one would expect to be grabbing hold of this debate – the congressional leadership, the major advocacy groups …just weren’t doing it …So when I started a lot of it was sort of trying to prod those people.”

That developed into writing some lengthy analyses of just why the numbers in the president’s plan just don’t add up, and deploying his readers to get every Member of Congress on the record as to whether or not they support privatization. Mr. Marshall scours the internet for comments given by obscure lawmakers to their local news outlets. He tags Democrats who may support the president’s the plan as members of the “Faint-Hearted Faction.” Republicans who oppose it – a group that, to Mr. Marshall’s glee, appears to be growing every day – make up the “Conscience Caucus.”

Mr. Marshall admits that the fight over Social Security has taken Talking-Points-Memo in a new, more activist direction. “It’s definitely far more focused than what I’ve done in the past … there is a much more participatory way that I’m approaching it.” But he stresses that he’s never presented himself as politically neutral. And part of the change, he says, is simply “experimenting with the medium, and what’s possible.”

Lately, mainstream news outlets have been treating the White House plan with the sort of skepticism that Mr. Marshall and others have long been calling for. And he’s happy to take a share of the credit: “I think the blogs have been pretty influential in this …,” he says. “Both before and during the campaign, there was this sort of alternative media created in the blogs …and it may be fair to say that on Social Security is where it really ended up having some sort of important influence.”