Estoppel prevents one from asserting a claim or right that contradicts what one has said or done before or what was legally established to be true.  Estoppel prevents the relitigation of issues.  It is a widely accepted principle that a valid final judgment on the merit bars a subsequent suit on the same cause of action by a party to the records. 

Usually, a state court finding that a party is estopped from maintaining an action is an adequate and appropriate state-court ground that supports the judgment which bars further review in the Supreme Court.  When the Supreme Court is concerned with a judgment placed upon two grounds, in which one involves a federal question, its jurisdiction is tested by inquiring whether the non-federal ground is independent of the other.  In Abie State Bank v. Bryan, 282 U.S. 765, 777 (U.S. 1931), it was opined by the Court that even if a party decides to comply with a law voluntarily, the party is not estopped from challenging its constitutionality.  The Supreme Court is bound to make an independent decision on the estoppel issue.