Contract law. Part of the actual, supreme law of the land.
SCOTUS has _no_ authority to amend USConstitution, at all.
Either SCOTUS finds USConstitution to be supreme over all politics, policy and unlawful law,
or SCOTUS is an enemy of USConstitution.
USConstitution, defend it, or you are an enemy of it.
We The People can lawfully, amend it using provisions, therein. Mere congressional _law_ or policy do not amend it.
Not the authority, surely. Unfortunately, they seem to posses the power.