"[5] During subsequent discussions with a US consular official, Terrazas gave conflicting answers as to whether or not he had truly intended to abandon his rights as a US citizen when he applied for his certificate of Mexican nationality.

Specifically, the court held that a law automatically revoking the citizenship of anyone who had voted in a foreign election was unconstitutional and unenforceable.

[11] The 7th Circuit Court of Appeals ruled that according to Afroyim v. Rusk, "Congress is constitutionally devoid of the power" to revoke citizenship;[12] and that Congress had no power to legislate any evidentiary standard for proving Terrazas's intent to relinquish his citizenship that fell short of a requirement of proof by clear, convincing and unequivocal evidence.

Rather, the court held that its 1967 ruling in Afroyim v. Rusk "emphasized that loss of citizenship requires the individual's 'assent,' ... in addition to his voluntary commission of the expatriating act" and that "the trier of fact must in the end conclude that the citizen not only voluntarily committed the expatriating act prescribed in the statute, but also intended to relinquish his citizenship.

Terrazas had argued and the 7th Circuit had agreed that the 14th Amendment, as interpreted in Afroyim, had left Congress without any constitutional authority to set the standard of proof for intent to relinquish citizenship at a level any lower than one of clear and convincing evidence.

The Supreme Court majority rejected this claim and held that Congress was within its rights to specify a standard of preponderance of evidence (more likely than not) when cases alleging loss of US citizenship were involved.

"Since we accept dual citizenship," he wrote, "taking an oath of allegiance to a foreign government is not necessarily inconsistent with an intent to remain an American citizen.

"[19] Justices William J. Brennan, Jr., and Potter Stewart argued that since Terrazas was born a dual US/Mexican national, his having taken an oath of allegiance to Mexico was consistent with his being a citizen of the U.S.

[23] On subsequent appeal, the 7th Circuit Court of Appeals reversed its earlier decision and, this time using a preponderance-of-evidence standard per the instructions of the Supreme Court, ruled against him, finding this time that there was "abundant evidence that plaintiff intended to renounce his United States citizenship when he acquired the Certificate of Mexican Nationality willingly, knowingly, and voluntarily.