Voter registration requirements in the USA are left to the individual states as long as they do not violate the Voting Rights Act of 1965 as amended.
Different states have different rules. Some allow felons to vote from prison, some =never restore voting rights. Some require an individual to petition for restoration. Some base re-enfranchisement on the crime.
In Connecticut, as long as the felon has completed his/her sentence and parole requirements he/she may register to vote and paid all ordered fines and restitution.
This means that the CONVICTED FELON Mayor Joseph Ganim of Bridgeport, who was sent to prison for stealing the taxpayers’ money and taking payoffs, was able to become an elector (registered voter) and then run for and win election as Mayor of Bridgeport.
In the 1950s and 60s Mayor Rado of Naugatuck was convicted and jailed for voter fraud twice. Each time after release and satisfaction of terms of his conviction, he got his vote back and the voters returned this felon to office.
If your states lets convicted felons vote and you oppose it, lobby your state legislators to change the law