It is significant that we take an oath to support and defend the Constitution and not an individual leader, ruler, office, or entity. This is true for the simple reason that the Constitution is based on lasting principles of sound government that provide balance, stability, and consistency through time. A government based on individuals — who are inconsistent, fallible, and often prone to error — too easily leads to tyranny on the one extreme or anarchy on the other. The founding fathers sought to avoid these extremes and create a balanced government based on constitutional principles.
The American colonists were all too familiar with the harmful effects of unbalanced government and oaths to individual rulers. For example, the English were required to swear loyalty to the crown, and many of the early colonial documents commanded oaths of allegiance to the king.