What is the history of a party promoting opposition candidates they deem to be unelectable?

I live in a smallish village (we’re not even a city), in the suburbs of Chicago. After the Mayor was indicted on poisoning us with contaminated well water for years & lying about it, 2 candidates emerged running for mayor. Towards the end, a 3rd candidate popped up. Turns out, he was paid by one of the candidates to run in an effort to siphon off votes from the other. In exchange, his son got an elevated position within the police department & god knows what else. And yes, the tactic worked & he won. And yes, years later, THAT mayor was indicted on federal crimes of bribery, & went to prison for a WHOLE month before getting released due to a supposed illness. And the new mayor? His son-in-law. If this much corruption goes on in a small town, nothing that happens in major parties would surprise me.

/r/NeutralPolitics Thread