In the end it is impossible to prove. But your point gets to the question of whether race is sui generis in some way. In terms of laws, we often do treat it that way.

I myself think that perceptions of class are actually often a more important divide for things like elections, or that they work in tandem with race.

New Hampshire voters have elected women executives before, but for most I'm guessing this is the first time they've been faced with the possibility of voting for a black one. So if there is any truth to this "effect" you might expect it to come into play here? Of course, some people think this "effect" doesn't exist -- follow the link to Gary Langer to see his argument.