The U.S. Constitution was always intended to be capable of modification. That's why we are able to enact amendments to it. That capability, however, does not, and never was intended to be, extensible to the dictates of the President. Theses days, anyone making noises about a "living document" or "the Constitution was never intended to be a suicide pact" has only one thing on their mind - the dissolution of the bedrock pact the governing has made with the governed. Period.
The single greatest step forward in the governing of a people was when the Romans overthrew their king and posted their constitution for everyone to see the agreed-upon limitations of their new government.