Partridge suggests that by extension of graft 'hard work' (which he says is an Australian expression) came to mean graft of the political kind (which is an Americanism) by being associated with illegal jobs (moonlighting?) I wonder if grift 'illicit profit (as from a con)' is related? Originally one of the meanings of graft (in the OED) is digging (which is hard work) related to the English word grave and the German word graben 'to dig'.