It may have been Oprah. Two professors gauge the celebrity’s impact:
We find her endorsement had a positive effect on the votes Obama received, increased the overall voter participation rate, and increased the number of contributions received by Obama. No connection is found between the measures of Oprah’s influence and Obama’s success in previous elections, nor with underlying local political preferences. Our results suggest that Winfrey’s endorsement was responsible for approximately additional 1,000,000 votes for Obama.
Remember that Obama’s final popular vote margin was a shade under 200,000.