The Principle of Double Effect is used to determine when an action which has two effects, one good and one evil, may still be chosen without sin. This principle is attributed to St. Thomas Aquinas, who used it to show that killing in self-defense is justified (Summa Theologiae I-II q64 art. 7).
With respect to voting, it would allow under certain conditions the toleration of the unintended evil of another for a proportionate reason. All four conditions must be satisfied:
- The action must be morally good, or indifferent, as to object, motive and circumstances.
- The bad effect(s) may only be tolerated, not directly willed.
- The good effect must be caused at least as directly as the bad.
- The good effect(s) must be proportionate to compensate for the bad effect(s).