Nothing stopping them from declaring that unconstitutional.
Um. It’s a ratified amendment. Good luck on that. An amendment would have to be repealed. Which can only be done by ratification of another amendment repealing it. See the 18th and 21st Amendment repealing it. And a ratification of a repealing amendment follows the same process as a standard amendment.
There’s four way to do this:
(1) Both houses propose an amendment with a two-thirds vote, and three-fourths of the state legislatures approve. Twenty-six of the 27 amendments were approved in this manner. (2) Both houses propose an amendment with a two-thirds vote, and three-fourths of the states approve the amendment via ratifying conventions. Only the 21st Amendment, which repealed Prohibition, was passed in this manner. (3) Two-thirds of the state legislatures call on Congress to hold a constitutional convention, and three-fourths of the state legislatures approve the amendment. (4) Two-thirds of the state legislatures call on Congress to hold a constitutional convention, and three-fourths of the states approve the amendment via ratifying conventions.
SCOTUS has never declared an amendment by itself to be unconstitutional. And an unconstitutional constitutional amendment doesn’t really exist by itself in US law theory.
Nonetheless. SCOTUS cannot repeal an amendment on its own. Period. All amendments must end it 3/4 (or 75% of states (ie THIRTY EIGHT states)) approving the repealment.
Again. In the words of Morgan Feeman, good luck.