This one seems to be inexplicably controversial.
But seriously, follks.
The right of all citizens to vote is what makes a democracy a democracy.
Or a republic, for all you pedants in the back.
It’s so much a part of the meaning that it’s in the word.
If you have citizens who are not allowed to vote, you’re not a democracy.
You’re something else, depending on what criteria you use to decide who gets to vote.
Plutocracy, probably; that one seems popular these days.
But there’s a more important reason than that for why the State should not be able to take away anyone’s right to vote.
It’s immoral for a society to prevent any of its members from a fair say in how its run.
TLDR: to judge a society or social design, you have to do it from a perspective of not knowing what place you will have within that society.
It’s basically a fancy philosophy version of the “what you do to the least of us, you do to me” dictum of you can find in the Christian bible.
And by any reasonable evaluation, a society where some members have no say cannot be fair.
And as we all remember from kindergarten, fairness is a fundamental guide to morality.
And morals aside …
Voting, in a democracy, is the most fundamental right: membership in a democratic society comes with the right to a fair and equal say in what that society should be like.
The most common reason why States take away the right to vote is as punishment for having been convicted of a crime.
Now, the State decides what is a crime.
The State also gets to choose what the response to conviction for a crime will be.
If we allow the State to take away a citizen’s right to vote because they’ve committed a crime then we are (effectively) allowing the State to disencranchise citizens for any reason it chooses.
That is not a good idea.
Seriously, that is just a really spectacularly bad idea.
And the norms of the governing class have kept us mostly away from that brink for a while but eventually that will fail too.