If I were king ...
I would require two forms of identification for voter registration.
1. an American Passport
2. evidence of residency in the voting district, driver’s licence, phone bill, etc.
At the polls on election days, I would require voters to present a photo ID with address.
After voting, they must immerse their thumb in a dye that does not fade until after the elections, to prevent multiple voting.
Absentee ballots would would only be issued to duly registered voters and be cross-checked for multiple voting.