The most typical scenarios of the spoiler effect involve plurality voting, our choose-one method. Plurality is extremely vulnerable to the spoiler effect so that even candidates with little support can act as spoilers. Here, we’ll give voter preferences in a simplified scenario.
% of voters Their ranking
“M1” is the Plurality winner here with the most votes (49%). Note, however, that 51% of the voters prefer “M2” to “M1”.
Here, candidate “m” changes the election outcome with just a small percentage of the vote. “m” does not win, but voters for “m” preferred “M2” as a second choice. So had “m” not been in the election, those voters would have voted for “M2” instead, giving “M2” the victory.