I don't think there are many people who vote for someone based on skin color or gender. I'm a woman, and I don't know any women who would do that.
The Republicans do not have the electoral votes in the right places in great enough numbers to win. The right is split by the Tea Party/religious conservatives versus more moderate Republicans. The youngsters are not voting for the right. Baby boomers are starting to die off from all the crap Americans eat.
The nomination is Hilary's to lose. I'm sure there is no end of dirt on her that could mess that up.
I would love to see a populist dark horse come out of nowhere, run on Facebook/Twitter, and disdain PAC money. If it ever happened, though, the person would probably be assassinated, so what's the use.