Very difficult in the US because the Federal government has no authority to require one, and the Supreme Court has consistently ruled that any regulatory or back door methods to effectively achieve a similar result is not legal.
It is one of the reasons ID resolution in the US is largely outsourced to private entities, which have no such restrictions.
> Very difficult in the US because the Federal government has no authority to require one,
It absolutely could require one for functions within the federally-regulatable sphere, and not prohibit its use for private and state functions.
(If it was national-but-decentralized, it could just set standards for the digital ID and let states issue them, the same way it has for physical ID, see, e.g., REAL ID.)
It is one of the reasons ID resolution in the US is largely outsourced to private entities, which have no such restrictions.