You do realize that's exactly what happened with the Gun Control Act of 1968 WRT firearms, right?
It shall be unlawful for any person to sell or otherwise dispose of any firearm or ammunition to any person knowing or having reasonable cause to believe that such person— (1) is under indictment for, or has been convicted in any court of, a crime punishable by imprisonment for a term exceeding one year;
There is nothing unconstitutional about making removal of voting rights an additional punishment for felons before they are convicted.
Ah, so we throw out the presumption of innocence? Or are you trying to say that because there aren't any
people who were convicted but still serving when it was passed that it somehow became magically Constitutional?
I'm sorry, but there's no statute of limitations
on a contraconstitutional act, it doesn't suddenly become constitutional because time has passed.
If the Gun Control Act of 1968 was applied retroactively to already convicted felons then it would clearly be an ex post facto law. If it was only applied to felons convicted after the date that the act was enacted then it would not be an ex post facto law.
Similarly, if a state passes a law prohibiting felons from voting and applies it retroactively to already convicted felons it is an ex post facto law. If the law is only applied to felons convicted after the law is enacted then it is not an ex post facto law.
All ex post facto means is after the fact.