No database? Can't cross reference social security numbers, passports? For SS they should know if you're a citizen based on what documents you submitted to get one.
@RecoveredExpert I never said it was the canonical data source, only that you could likely find many of these people in it and be able to determine their status. For SS it would be which forms they filed
@feld another partial dataset covering only a subset of people allowed to vote is not solving the problem that you quoted in your initial post. There is no single dataset to allow for authoritatively determining voting eligibility in the US and that is by design. Not saying that I agree (or disagree) with the reasons for that.