Perhaps the most interesting work I've seen in this election is this study (PDF) by University of Maryland economists Craig Garthwaite and Tim Moore, who studied subscriptions to O! (Oprah's Magazine) and purchasing patterns in books recommended by Oprah's Book Club to conclude that Oprah Winfrey had a major -- and possibly even decisive -- impact on the Democratic primaries, netting Barack Obama more than one million votes.
I've read -- well, skimmed -- the whole paper, and it strikes me as exceptionally thorough. The thing you worry about when you are conducting one of these sorts of analyzes is that there is some latent effect that you can't capture well through other variables -- some piece of DNA that might not manifest itself in traditional demographic categories like age, income or race, but which simultaneously makes one both more likely to subscribe to Oprah's products and to vote for Barack Obama. Garthwaite and Moore do everything they can to identify such latent variables, by (for example) searching for an Oprah Effect in the 2004 Illinois Senate race (there wasn't one) and also looking at subscription patterns of other popular women's magazines.
It's pretty convincing work. And it's important work too, because endorsements are a notoriously slippery thing to study. If you come right out and ask people whether endorsements matter, they are likely to tell you 'no'. But I doubt that people are being entirely honest. It's not that they're consciously lying to pollsters -- they're lying to themselves. If you stood outside of a Best Buy and surveyed everyone who had purchased an iPod, and asked them whether their purchase decision was influenced by iPod's ubiquitous advertising, I doubt very many of them would say yes. But the reality of the situation is that Apple's slick advertising and precise branding are hugely responsible for their dominance in the MP3 market, where they succeed in selling at a high price point in spite of a product that is not all that strongly differentiated from its competitors.
As a matter of pride, people are going to resent the implication that they aren't capable of making their own decisions. And certainly, Oprah doesn't hold anyone in a hypnotic spell (her ability to get hordes of people to purchase crap like A Million Little Pieces notwithstanding). But she might get people to get up off the couch and start becoming more engaged in the election, and when they do, to perceive things through a somewhat Obama-friendly lens. The researchers found that Oprah's involvement also increased votes for the other Democratic candidates -- just not as much as for Obama.