With software I'd be wary of making it the last line of defense, especially for very significant acts such as voting. A bug in the system would be totally fatal

With software I'd be wary of making it the last line of defense, especially for very significant acts such as voting. A bug in the system would be totally fatal, not only for society but especially for the victim. Even more so if bugs aren't commonplace but only sporadic. Using cryptography in addition to "classic" means of attestation sounds a lot better to me. You could also make it an "optional feature" of people's citizenships (like 2FA on an account) to ease adoption.