The founding fathers of the United States of America state in the Declaration of Independence that,
"...Governments are instituted among Men, deriving their just powers FROM THE CONSENT OF THE GOVERNED, that whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or abolish it, and institute new Government..." [emphasis mine]
Thus, our very founding document requires a recognition that states had the choice to join the union, and they have the choice to depart from the union.
The president should sign an executive order publicly recognizing this fact and requiring all officers and agencies of the executive branch to recognize, and not to oppose, any such efforts.