"Laws on ex-offenders not being allowed to vote are more open to argument about legitimacy, but I don't know whether the US does this or not." Grab

It may vary by state, Graham. In Alaska, ex-offenders in most cases are eventually reinstated. The exceptions are 'crimes of turpitude', and I never have had a clear idea of what that refers to.

The other category of being denied the vote is not being registered. I used to work at the Division of Elections and it is surprising how many people assume that they can walk in on Election Day and vote. "I'm a US citizen!"

So far as I know, every state requires registration, although some allow it up to and on Election Day. Other states, like Alaska, enforce a time period of 30 days before the big day.