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CA voter registration lacks integrity. It relies solely on the person's honesty when signing the affadavit that they are aUS citizen and that all information is correct of the registration form. CA like other states consider the statement signed under threat of perjury as verification of citizenship.
CA Elec Code § 2111 (2017)
A person may prove he or she is a citizen by his or her certification under penalty of perjury on the affidavit of registration.
CA Elec Code § 2112 (2017)
Notwithstanding any other provision of law to the contrary, the fact that a person certifies to his or her United States citizenship by signing his or her affidavit of registration shall be deemed evidence of citizenship for voting purposes only.
https://law.onecle.com/california/elections/2111.html