This morning on Morning Joe,(they are political animals over there) had a poll up that answers your question.
Ben Carson beats Hillary the best, by about 10 points, and Trump beats her by six, Jeb Bush even beat her by a couple of points.
They all beat her.
Carson at the top of the GOP ticket would prove fatal to the Dems, at all levels.
Think for a moment what states would be in play for the GOP that almost never are, if the GOP ran a candidate who could keep most social and economic conservatives in the fold, but peal a mere 20% of African American voters from the Dems? And Carson could pull 30% without hardly trying. I would not be surprised, if Carson gets the nomination, that he might pull closer to 40%. Crazy, I know, but possible when you look at who will most line up with in values with the religious African Americans, but who can in no possible way be confused with a rich white, racist guy. Further, do you think Carson versus Hillary would even be a contest? She might lose 48 states to him.