> Racism was the historic angle-- but even if you target a black community and make felons of all the men, they would only comprise a minority of voters in a white district, and it just reduces the number of all eligible voters in an all-black district.
Reducing the number of voters by eliminating a disfavored class from representation while retaining the political power of apportionment that counts the disebfranchised citizens was the whole point of the 3/5 compromise, and mass criminalization plus felon disenfranchisement is its direct replacement.
Also, you maybe want to consider the effect on a mixed-race district where, say, a sufficient minority of whites are sympathetic to black interests that together they would, before selective disenfrnachisement, constitute a majority.
> Is this a local politics thing (school funding, etc.)?
That, too, but its largely a federal politics thing.