This NY Times article on mental illness and the right to vote shows some different motivations people have for wanting to remove the Constitutionally-granted right to vote from some people with mental illnesses.
Some people pretty clearly don’t want mentally ill people to have political influence (crazy people are irresponsible and dangerous, omg, etc.). Your standard (and boring) ignorance, prejudice, fear. (On a side note, I think it’s interesting that the usual fear – physical danger – is replaced here by something non-physical that seems to carry the same emotional intensity. I wish I knew how exactly they’re worried about some mentally ill people misusing their vote. Vote for the opposing Presidential candidate? Raise or lower property taxes? Revoke leash laws?)
A separate set of people seem to want to restrict the right to vote because they are worried that people who are not competent will be manipulated by others into voting accordingly. That does concern me some – it makes me think of facilitated communication, in which children with autism who lacked the ability to communicate were basically used as Ouija indicators by practitioners who (often entirely unintentionally) inserted their own expectations and neuroses into the output, which in some cases resulted in false accusations of sexual abuse, among other things. So you do have to have some mechanism for preventing abuse.
Phrasing this like it’s about mental illness makes it sound like it’s a mental illness issue per se. And it’s not. It’s something that happens in mental illness sometimes, but not always. What it is, is a competency issue. They mention the elderly in the article, for example. Some, but not all elderly are not competent to make decisions for themselves. Some people are temporarily or permanently disabled after a stroke and may not be competent to make decisions for themselves. And on and on.
The point is that if voting is about being mentally competent enough to not be manipulated by someone else, that’s what it’s about. Like how many traditionally male, or traditionally female jobs aren’t (or shouldn’t be) about having to meet certain requirements only if you’re the wrong gender – they’re about everyone having to meet the same set of requirements to perform to job adequately.
And then either you get a consistent law (that probably won’t get consistently applied, but oh well) or you get a lot of people up in arms because it was okay to have that competency requirement when it was just for crazy people, but it’s not okay to try to apply it to them. (There are a lot of old people, and they vote.)