I get that this is a tech site, and we like to think that everything is the result of smart hackers (even the Trump election! because some Russian hacker could be the only explanation), but there are low tech options here as well.
The Project Veritas videos, whatever you may think of them, had high level operatives openly discussing tactics they use to affect elections. A few extra paper ballots seems like something a motivated person with access to a photocopier could accomplish.