Best effort? You haven't met my mail carrier, who was busy one day and decided to remedy that by skipping delivery to my neighborhood that day, marking my package "no such residence at address," returning it to its sender, who just happened to send it from another country.

So, yeah. I'm pretty sure it's a least-effort-without-getting-fired delivery protocol.

I would almost certainly trust UDP more to actually deliver my ballot, forgetting things like security, etc.