Penn is the best-selling author of Microtrends: The Small Forces Behind Tomorrow's Big Changes. This book mirrors his approach to elections: identify influential archetypes, like 1996's Soccer Moms, and fine-tune campaign proposals (for example, requiring parental-control "V-chips" in all new TVs) to win them over.

Microtrends applies this analysis to the social sphere: Penn's "microtrends" involve tiny but visible segments of the population who are growing in size or influence due to social and economic forces—like the 1 percent of California youths who list "sniper" as a career goal.

The book's critical reception was mixed. While it gained some corporate cachet, political writers showed Penn no love. Influential blogger Ezra Klein called Microtrends "so epically awful as to take the entire polling industry down with it." Maureen Dowd snarked that "the chapters all read like reports that Mr. Penn wrote for clients," and compared the names he gave his population segments—among them Shy Millionaires, Mildly Disordered, and Uptown Tattooed—to "a new lineup of Fox reality shows."

But perversely, Obama's success has vindicated Penn's microtargeting approach. This year, microconstituencies that could easily have fit Penn's book were poised to make a huge splash in the relatively small Democratic primary pool, swelling 2004's turnout of 16 million by 80 percent.

(Primary and caucus voters are a small, skewed fraction of those who cast ballots in November, estimated at 122 million voters in 2004 and certain to grow this year.)

Receptive to messages of inspiration, change and political reform, groups like newly energized Internet donors, nostalgic middle-aged liberals, freshly recruited pundits, and atypically upscale independents have provided Obama's keys to success.

Online Donors.

Foremost among the influencers are a new wave of political donors, who have always been a microconstituency. Most Americans donate to charitable causes like churches, medical research, even pet rescue—not political candidates. The total number of donors to all presidential candidates in 2004 totaled just one million—less than 0.5 percent of the adult population. But the Web has caused this smallish universe of political donors to rise sharply: Obama's backers now claim that, if nominated, he alone will attract two million donors by November.

These new donors' influence goes far beyond the attitudes changed by the ads they fund. They have rewritten the rules about how quickly a candidate can capitalize on sudden momentum, and pumped previously unseen amounts of cash into the campaign industry. These new online donors are perfect specimen of a Penn microgroup: small, but growing rapidly as a result of new outside forces (the Internet, here).