I know...its only one data set, but I still think its interesting...It appears that Alan Greenspan and a voters own minister have the only real positive impact on voters according to this PEW data. (September 20-07) Much more info at the link...Oprah et all are discussed.
"...Former Federal Reserve Chairman Alan Greenspan is one of only two individuals tested in the poll who had a net positive impact on potential voters. While 65% of the public said an endorsement from Greenspan wouldn't affect their voting choice, 18% said it would make them more likely to support a candidate (14% said less likely). Republicans are more likely than Democrats to see a Greenspan endorsement as a plus.
Religious leaders have a positive impact on voters as well: 18% say an endorsement from their minister, priest or rabbi would make them more likely to vote for a candidate (12% say less likely). Clergy influence is especially potent among Republicans. Fully 27% say they would be positively influenced by a clergy endorsement. This compares with only 17% of Democrats..."
http://people-press.org/reports/display.php3?ReportID=357