The Preamble to the United States Constitution states:
“We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.“
Note the use of “Constitution for the United States of America” and not “Constitution of the United States.”
From the Fourteenth Amendment’s mention of “Constitution of the United States,” it is argued that the original intent of the Founding Fathers was subverted in referring to a Constitution of the United States as a single entity, as opposed to a Constitution for the United States of America as a union of sovereign independent states.
However, if you look at the context, it’s specifically referring to the oath of office taken by all officers and legislative members of the States, and all officers and Congressional members of the United States, which includes the President of the United States.
From the original Constitution, the presidential oath or affirmation of office is:
“I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.“
Therefore, it is clear from the Constitution that the Founding Fathers who signed it interchangeably referred to both a “Constitution for the United States of America” and a “Constitution of the United States,” without drawing any particular distinction between them.