Barack Obama’s run for the White House has already made history, and not merely because of his race. He came out of nowhere to upset the supposedly unbeatable Hillary Clinton. Now the nomination seems within his grasp. Can he win in November? That may come down to whether he can win in Missouri. Missouri is one of the nation’s leading bellwether states; it has picked the winner in every presidential contest since 1900 with the exception of 1956, when state voters went for Adlai Stevenson rather than President Eisenhower. For Obama, taking Missouri will be tough, but not impossible. The...