When I got my passport renewed this year I paid an additional $30 for a US Passport Card. The mechanism exists for a national identity card. This is a picture ID card proving citizenship -- this should be provided to all citizens over 18 years of age for a nominal fee like a state id or driver's license. This Passport Card proves USA citizenship and the right to vote -- that person's name would be on the voter registration roll for the jurisdiction -- case closed -- have a nice day.

Other forms of ID DO NOT PROVE CITIZENSHIP.