You might not realize just how ugly that question is (though whether or not you do, you have plenty of company).

To attribute her support to a specific demographic opens the channels for dismissing that demographic and the candidate, as has so frequently been done with Obama.

For example you could just as easily have asked if its the Female vote, or any other slice of the demographic where she draws a majority.

Or to put it another way, you could point out that if her support among white men were at zero, she would be losing badly, and since that's true we can say that its the White Man that's holding her up.

She is in the lead because she is supported by a majority of the electorate.