As we mark the national semiquincentennial on July 4, we might well reflect on Benjamin Franklin’s answer to Elizabeth Willing Powel, when the elderly sage left the Constitutional Convention in 1787 and the Philadelphia matron demanded, “Well, Dr. Franklin, what have we got, a republic, or a monarchy?” To which Franklin famously responded, “A republic, if you can keep it.” That brief riposte underscores a basic fact about these United States: that we are a democratic experiment in republican...