Our results suggest that Winfrey’s endorsement was responsible forapproximately 1,000,000 additional votes for Obama.
My personal thoughts:
Opinion leadership is very important, but I'm hesitant to think that people are forming opinions just because Oprah is making one. A big factor, as I see it, is the increase in voter participation and voter awareness: getting people to recognize think that the election is important is probably more important than the direct endorsement.
Caution, though: I haven't actually, ya know, READ the paper.