granny farming
n.—Gloss: a form of voting fraud in which pre-marked absentee ballots are taken to nursing homes to be used and mailed by residents. «“Granny farming” is a term for visiting retirement homes with pre-marked absentee ballots.» —“The Buzz: Get stumped” by Kyle Odegard Gazette-Times (Corvallis, Oregon) Nov. 6, 2007. (source: Double-Tongued Dictionary)
I recall hearing this term being used back in the late 80s in the UK – the Tories had a fondness for using the tactic in local elections.