Yes, we know republicans have been caught cheating via ballot harvesting.... By actually not turning in the ballots collected from neighborhoods voting for their opponent....
There are different potential ways to cheat in ballot harvesting....but it is not due to the voter, voting....they are most all legit registered voters who requested a ballot to be sent to them at their homes and they legitimately voted. The person collecting them is most always legitimate workers for the campaigns and simply drop the ballots off in mail or ballot drop boxes or delivered to precincts....
But the cheaters find ways to cheat....usually for LOCAL and State elections where fewer ballots are needed to make an impact on their election and they seem to be mostly insiders who have the access to cheat, ...but they also seem to get found out and caught.
This is why most states limit who can turn in ballots for you...now a days depending on the State...it is usually limited to a relative. My husband turns in my ballot on election day, when he goes in to vote in person.