It doesn't need to be at scale if the elections are won by very small margins. Successfully executing voter fraud on a few key districts can sway an entire election.
Even in fptp systems like the UK and the US correctly predicting the precise polling stations to infiltrate and compromise to stuff the required 1000-100000 ballots is tricky. And in the UK your whole plot would probably fail anyway - as random people and representatives of the other parties would be present observing you.