Very common misconception. Commonly believed right here on FR in fact. However, totally wrong. Only fourteen states require any sort of process or request to have voting rights restored after completion of all supervised release, upon completion of parole or simply upon release from prison. The other thirty four have automatic restore provisions. There are two states, Vermont and Maine, that allow voting from prison.
However, totally wrong.
No, it is totally correct to argue that States have the absolute right - settled law - to exclude certain classes from voting.
I have so filed and argued exactly that without any plausible retort.