"We all carry around implicit attitudes based on how we have understood the way the world works as we grow up and if you never see a certain kind of person in a certain job, and all of a sudden somebody who looks or sounds like that shows up, you go 'Oh, wait a minute. Who are you? You don't belong here,' " she said.

She said she believes this is a factor in some of the criticism of Clinton, who was First Lady to former U.S. president Bill Clinton when Campbell first met her at the G7 Summit in Tokyo in 1993.

"Things that people hate about her ... are kind of small potatoes, compared to other people who have run for public office," said Campbell, who then mentioned the email scandal and the attack on the U.S. embassy in Benghazi.

"You're looking for the way to be able to say 'Aha! You see? That's why I'm not going to vote for her.' But underlying it is really this sense of discomfort that 'I've never voted for somebody like that for president.' "