XKCD has this very funny bit on the security of electronic voting machines. There is a very interesting lesson here, but probably not the one you think. Think about these questions:
Why is there such a hyper-focus and concern about the security of the electronic voting machines when most states don’t even require an ID to vote? Why are we concerned about someone hacking a voting machine when states accept absentee ballots with absolutely no verification of the sender? And has anyone looked into the software that is used to tally and report votes after Election Day?
The point is that security must be about the total life-cycle of the process, not just a specific point in the process. The security of the electronic voting machine is important. But without a reliable way to scrub the voting roles of those no longer eligible to vote, identifying the physical person casting the vote or submitting an absentee ballot, and tallying and reporting the vote after Election Day, you won’t have a secure process.
I don’t believe that there has never been a documented case of electronic vote tampering. That doesn’t mean that it hasn’t or won’t happen. But if you are looking for the most likely place for voting fraud, the inside of the machine is not it.
Close elections have been decided by the cemetery vote long before electronic voting.