Oprah's political endorsement value

[via Marginal Revolution] A study on the value of Oprah's political endorsement to Barack Obama concludes that "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." They also note that "the results ... suggest that Winfrey’s endorsement was responsible for approximately additional 1,000,000 votes for Obama"

The paper can be read as a PDF.

I cannot tell if 1MM incremental voters is valuable, though it feels like a strong number.

My curiosity is stoked: what are the five most valuable endorsements a candidate can receive? Labor unions? Governors? Senators? Newspapers? Which ones?