Here's some crosstabs from the latest Quinnipiac poll matchup between Hillary Clinton and various GOP candidates, in this case, Ben Carson.
Clinton loses pretty badly to Carson, but the reason why is she loses the white vote badly. Among white women it's 54-40% Carson, but among white men it's 60-29%.
Even Obama got more of the white vote than this in 2012.
Hillary's numbers among white voters are terrible, but she doesn't break 30% against any of the Republicans among white men period, not against Carson, Bush, Fiorina or Trump.
Keep an eye on that number should Joe Biden get in the game.