As always it is best to let’ a member of the Magestrium to explain:
"Consider the case of a Catholic voter who must choose between three candidates: candidate (A, Kerry) who is completely for abortion-on-demand, candidate (B, Bush) who is in favor of very limited abortion, i.e., in favor of greatly restricting abortion and candidate (C, Peroutka), a candidate who is completely against abortion but who is universally recognized as being unelectable.
"The Catholic voter cannot vote for candidate (A, Kerry) because that would be formal cooperation in the sin of abortion if that candidate were to be elected and assist in passing legislation, which would remove restrictions on, abortion-on-demand.
“The Catholic can vote for candidate (C, Peroutka) but that will probably only help ensure the election of candidate (A, Kerry). Therefore the Catholic voter has a proportionate reason to vote for candidate (B, Bush) since his vote may help to ensure the defeat of candidate (A, Kerry) and may result in the saving of some innocent human lives if candidate (B, Bush) is elected and introduces legislation restricting abortion-on-demand. In such a case, the Catholic voter would have chosen the lesser of two evils, which is morally permissible under these circumstances.”
Bishop Rene Gracida