I have a very good friend in Seattle. Some 40 years ago, he lived in a big Midwest city...I won't name it but it used to be one of our biggest, if not the biggest, industrial city in the US. Far from it today.
He had a girl friend who worked for a community advocacy group. You could call her a community organizer. Her thing was the Mayoral elections. She would canvass the declining neighborhoods where there were homeless people, addicts, gamblers and street people. She would register them to vote, each individual several times, using variants of their legal names. The addresses for their residences would be a homeless shelter or some community services facility. She would get absentee ballots, which then would never reach the registered voter but would arrive at the listed address. Then, on election day, she would go to the poll late at night and in his words “stand there for an hour or two feeding ballots into the machine.” And of course she was not the only one doing this. He said “you'd go to bed at 10:00 and candidate A had a strong lead but when you got up the next morning, B had come from behind to win the election. And if you look at the precincts that put him over the top, it was the blighted neighborhoods and burned out abandoned areas. The city hasn't had a Republican Mayor since 1962.
I guess this is what is meant by “ballot harvesting.” She wasn't his girlfriend very long.
He says today, in Seattle and Portland and San Francisco and Los Angeles he knows the same thing is going on, on a larger scale. He insists this is why those cities want homeless people.