Jeb Bush led the Republican field in the most recent Wall Street Journal/NBC News poll, but the data show significant challenges ahead for him.

In his 2016 announcement speech, Mr. Bush noted how the U.S. is a land of opportunity, a country where anything is possible. “Great things . . . can really happen. And in this country of ours, the most improbable things can happen as well. Take that from a guy who met his first president on the day he was born, and his second on the day he was brought home from the hospital.” Improbable, yes–but such a feat is a lot more improbable for Americans from any other family.