Later this week, the historic nomination of the first female candidate for president by a major political party at the Democratic National Convention in Philadelphia is sure to generate considerable hoopla. But, as with all U.S. presidential conventions in recent decades, the outcome is already certain.

Such predictability was not always the case. In fact, three-quarters of a century ago, the City of Brotherly Love played host to a very different convention—one whose outcome was so unexpected it became known as …