Comment by deathanatos

Just to add to this, not only does the federal government have, for example, SSNs, but, e.g., in my state, supplying a SSN (which then gets verified with the federal government by the state government) is one of the ways in which you can prove citizenship for the purposes of registering to vote.

There's also birth certificates, which are state-based databases, AFAIK, so there is that route too, and that process is more along the lines of how we're a federation.

(& for voter registration, probably the most common method, at least IME, is to bootstrap that from your state license/ID; since those are generally REAL IDs now, most individuals would provide birth cert and SSN, since that is the most broadly fulfillable requirement of REAL ID.)