P was born in the United States, the son of a Mexican citizen. P acquired at birth both United States and Mexican citizenship. In the fall of 1970, while a student in Monterrey, Mexico, at the age of 22, P executed an application for a certificate of Mexican nationality, swearing 'adherence, obedience, and submission to the laws and authorities of the Mexican Republic' and 'expressly [renouncing] United States citizenship, as well as any submission, obedience, and loyalty to any foreign government, especially to that of the United States of America. . . .' The certificate recited that P had sworn adherence to the United Mexican States and that he 'has expressly renounced all rights inherent to any other nationality, as well as all submission, obedience, and loyalty to any foreign government, especially to those which have recognized him as that national.' P read and understood the certificate upon receipt. A few months later, proceedings were instituted to determine whether P had lost his United States citizenship by obtaining a certificate of Mexican nationality. P denied that he had. The Department of State issued a certificate of loss of nationality. The Board of Appellate Review affirmed that P had voluntarily renounced his United States citizenship. P brought this suit against D for a declaration of his United States nationality. Afroyim v. Rusk held that the 14th Amendment ''[protects] every citizen of this Nation against a congressional forcible destruction of his citizenship'' and that every citizen has ''a constitutional right to remain a citizen . . . unless he voluntarily relinquishes that citizenship.'' The court held that a person of dual nationality 'will be held to have expatriated himself from the United States when it is shown that he voluntarily committed an act whereby he unequivocally renounced his allegiance to the United States.' It held that P had taken an oath of allegiance to Mexico and had 'knowingly and understandingly renounced allegiance to the United States in connection with his Application for a Certificate of Mexican Nationality.' The Court held that D had 'proved by a preponderance of the evidence that P knowingly, understandingly and voluntarily took an oath of allegiance to Mexico, and concurrently renounced allegiance to the United States,' and that P had therefore 'voluntarily relinquished United States citizenship pursuant to § 349 (a)(2) of the . . . Act.' The Court of Appeals reversed. It ruled under Afroyim v. Rusk that Congress had no power to legislate the evidentiary standard contained in §1481 (c) and that the Constitution required that proof be not merely by a preponderance of the evidence, but by 'clear, convincing and unequivocal evidence.' D appealed.