A new study suggests Oprah Winfrey’s backing of Barack Obama was beneficial for the presidential candidate in an indirect way.

“The Oprah Factor,” a just-published paper by Andrew Pease and Paul Brewer of the Department of Journalism at the University of Wisconsin-Milwaukee, describes a November 2007 experiment aimed at measuring the impact of the talk show host’s endorsement. One group of students read an Associated Press story about the Obama campaign that did not mention Winfrey, and another read an expanded AP story that included four paragraphs about her plans to campaign for the candidate.

Afterward, members of both groups were asked four questions: What is your overall impression of Obama? How likeable or unlikable is he? How likely do you think it is that he will win the nomination? And if the election were tomorrow, how likely would you be to vote for him?

The two groups’ ratings of Obama’s favorability and likability were essentially the same. However, those who read about Winfrey’s endorsement rated Obama’s chances of winning more highly than those in the control group and said they were more likely to vote for him.

The Oprah endorsement suggested to students that Obama was a more viable candidate than some expected, and this in turn increased the likelihood they would vote for him. That suggests at least some of the participants favored Obama but were holding back for fear he didn’t have a serious chance at the nomination. Winfrey’s high-profile endorsement eased those fears.

The authors are cautious about drawing broad conclusions from this research. They note that Winfrey’s endorsement may be more impactful than that of most celebrities, given her unique status as a cultural icon. Nevertheless, their survey points to a new, nuanced way to view the phenomenon of celebrity endorsements.

From one perspective, “our findings could be interpreted as illustrating the trivialization of the campaign process,” they note. On the other hand, if voters are making the calculation that a celebrity endorsement makes a candidate more likely to win, and they prefer to cast their ballot for someone who has a real chance of success, this “may reflect strategic reasoning rather than starry-eyed susceptibility on the part of citizens.”

Sounds like a good discussion topic for the next Oprah Winfrey Show.