In 2006, Matthew Baum, associate professor of political science at UCLA, studied "the Oprah effect" in the 2000 presidential race, looking at how politicians' appearance on TV talk shows affected people's voting patterns.
This is what he found, from a UCLA article:
"The Oprah Effect" had more than twice as strong an impact on the voting patterns of politically unengaged voters as traditional news coverage had on the patterns of the politically engaged."
"Because highly engaged voters already possess substantial political knowledge, exposure to campaign coverage via either the soft or hard news media is likely to have a limited effect on their voting choices," he said. "They're wading through party mailers, visiting candidates' websites, debating issues with friends and maybe even attending candidates' forums. They've got information coming at them from all directions, so no one source is overwhelmingly influential."
So, an endorsement by Oprah, the queen of talk shows herself, will have tremendous sway on politically unengaged voters who lack substantial political knowledge; this conclusion doesn't bode well for Hillary Clinton.