The democratic principle should require that within an appropriate electoral area each person should have one vote. Any deviation from this principle requires a precise and equally principled justification. I can only see two main acceptable restrictions on such franchise rights. One that a person is considered to be too young to vote. As we should err on the side of caution in withholding franchise rights, I would like to see people enfranchised from the date of their 16th birthday. A second restriction on franchise rights needs to apply to those who have a serious mental condition such that they can not exercise a choice and there is a danger that persons they are dependent upon will exercise their voting rights in their place.
If we add to this list of restrictions, we need to be able to establish a solid and principled case. To say that prisoners who have been convicted of particularly nasty crimes should be excluded, is not a good argument in itself as it merely appeals to popular feelings of disgust for them. But to exclude people from the franchise because we judge them to be evil is a worrying approach. It could lead to different people wishing to exclude a variety of categories of the electorate by using arguments which are racist, xenophobic, sexist, anti-capitalist or what you will.
Furthermore it is not just prisoners who are currently excluded from the franchise. In the United Kingdom the ban includes those who are settled here from overseas nations other than Ireland or Commonwealth Nations. Then there is the lack of attention we pay to the efficiency of the electoral registration system. Millions who qualify are missing from registers. These involve a high percentage of the homeless, the rootless, ethnic minorities, the poor and the young. An extra reason for votes at 16 is that those who are due to attain the vote at that age could initially be placed on electoral registers at school when they are"attainers" at the age of 15. If we achieve 100% electoral registration for 16 year olds, it will be easier for the registration system to keep people on registers as they grow older.
Are there no people who should be excluded from electoral registers outside of the two main categories I mentioned in my opening paragraph? There are only two more that I can think of. First, under the principle I am arguing from there is no need for people to register in more than one place at the same time and then choose which of their franchise rights they use on any particular occasion. And if people register in more than one place, there is no automatic check to ensure that they have only used their franchise the once at any one time. Secondly, there is no case for British Citizens settled overseas having the vote in this country. Under the democratic principles I am arguing from, they should be given the vote in the country in which they are resident. We should support any of their efforts to achieve these rights in their country of settlement.
If my arguments are to be challenged, then I hope that critics will either seek to show how their arguments fit in with democratic principles. It is, of course, possible to argue against the democratic principles which I have used on fascist, stalinist, elitist, populist and other non-democratic grounds. We should judge the opposition to "votes for prisoners" to see which of these perspectives they fit into.
Of course, democracy requires much more than democratic voting rights. But it is seriously hampered without them.