Also, think about the demographic make-up of America’s penal system, and you’ll find another reason Republicans don’t want prisoners to vote.

Pretty sure that is the actual reason.

• Lose civil war. Have to free your black slaves.

• Create laws and law enforcement culture that disproportionately impact black folks.

• Steadfastly pretend you have done neither of those things.

• Lock 'em up!

• Sorry, prisoners can't vote. Really, our hands are tied. Someone think of the children.

Closest thing to a confession I'm aware of:

“The Nixon campaign in 1968, and the Nixon White House after that, had two enemies: the antiwar left and black people,” former Nixon domestic policy chief John Ehrlichman told Harper’s writer Dan Baum for the April cover story published Tuesday.

"We knew we couldn't make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana and blacks with heroin, and then criminalizing both heavily, we could disrupt those communities. We could arrest their leaders, raid their homes, break up their meetings, and vilify them night after night on the evening news."

"Did we know we were lying about the drugs? Of course we did."

https://www.cnn.com/2016/03/23/politics/john-ehrlichman-richard-nixon-drug-war-blacks-hippie/index.html

https://www.businessinsider.com/nixon-adviser-ehrlichman-anti-left-anti-black-war-on-drugs-2019-7

Here's older evidence that goes back to much earlier.

Marijuana was Anslinger’s golden ticket. He used his office to trumpet the association between weed and violence, so that it could be criminalized. “You smoke a joint and you’re likely to kill your brother,” he was known to have said. McWilliams explains that in this effort, “Anslinger appealed to many organizations whose members were predominantly white Protestant.”

From the beginning, Anslinger conflated drug use, race, and music. “Reefer makes darkies think they’re as good as white men,” he was quoted as saying. “There are 100,000 total marijuana smokers in the U.S., and most are Negroes, Hispanics, Filipinos and entertainers. Their Satanic music, jazz and swing result from marijuana use. This marijuana causes white women to seek sexual relations with Negroes, entertainers and any others.”

https://timeline.com/harry-anslinger-racist-war-on-drugs-prison-industrial-complex-fb5cbc281189

When one thinks too hard about it, the whole "we can't trust prisoners to vote, they'll vote to legalize crime!" argument is pretty ridiculous too. Criminals don't just universally love crime, like a guy in for having weed isn't likely to want to legalize murder or something, he doesn't want to be murdered or see people murdered any more than anyone else, and anyone actually crazy enough to want to legalize that is going to be an irrelevant minority of the vote anyway (prisoners in general will be a minority, unless most of your population is locked up, in which case you either actually do have way too many crimes that shouldn't be illegal, or your society is so terminally dysfunctional that it's laws won't much matter anyway).