Marines do not serve the President, we serve the Country and the Constitution.
What branch of service were you in, sir?
None, I suspect.
All command authority flows from the CIC. As far as serving the Constitution, check Article II.
When Hitler assumed power in Germany, one of the things he did was have the military re-swear their oaths to (in order): (1) Adolf Hitler himself, (2) Germany, and (3) German people. This completely changes the equation when those holding access to deadly force are sworn to do it for Dear Leader and not, as in America's case, its Constitution.
If this should ever happen, Obama has declared open war on America and its people.