You are deliberately equating two issues for the benefit of your argument. One is the issue of abortion itself. The other is the issue of voting. Clearly these issues are closely related. But they are not identical. While abortion itself is “inherently evil”, voting for someone who has the weaker position on abortion may not be. So by declaring that some voting decision is in the realm of “prudential judgment”, that is not exactly the same thing as declaring that the issue of abortion is up for “prudential judgment”. I can demonstrate that by considering something other than abortion that is also “inherently evil”.
Consider that homosexual acts are inherently evil. That is not open to prudential judgment or extenuating circumstances. Now suppose you have an election where candidate A favors severe punishment for anyone caught in the act of homosexual behavior, even in private. Candidate B favors making this act not a crime at all, unless it is done in public. Neither candidate favors gay marriage or any other recognition of homosexual relationships. Their only difference on this issue is over the question of punishments. But suppose there is also some prudential judgment reason that you have to personally prefer candidate B for other reasons. In every other respect (including abortion) these two candidates have exactly the same position.
If it were really true that voting for someone with a position on an inherent evil is the same thing as choosing that evil directly, then one could not vote for candidate B in this election, even though both candidates have positions that by today’s standards sound very conservative. If you still think that candidate A must be preferred over candidate B, just consider sliding their positions on homosexuality more and more to the extreme side, but always keeping candidate A’s position a little more strict than candidate B’s (like A prefers 30 years in prison while B prefers only 25 years for homosexual behavior). If prudential judgment may never play any role in the voting decision, then you would have to always vote for the candidate with the stricter position, no matter how unreasonably strict it was! That is the unavoidable consequence of declaring as a firm principle that voting is exactly the same thing as doing. Prudential judgment obviously is legitimate in voting, if only to the extent of deciding what level of strictness is strict enough.
Similarly, Ender wrote:
This is not really the question. By claiming that there are proportionate reasons to vote for a candidate with the weaker position on abortion, that is not equivalent to claiming that the issue of abortion itself is proportionate to the other issue for which you base your vote. An example can be constructed similar to the one above. Suppose two candidates both oppose abortion and want to make it illegal, but one favors a slightly harsher punishment for it. It is easy to see that if the difference between the candidates is small enough, one could justify voting for the one with the weaker position just because he also favors improving public parks that you like to visit. This does not mean that abortion is proportionate to public parks. It just means that when the difference is small enough, other factors may be considered.
Of course most elections today are not about such small differences on abortion policy. The elections are usually between a candidate who favors continued legalization, and maybe even subsidization of abortion, while the other candidate favors making it illegal. But the fact that the example exists shows that some judgment is needed to decide when a difference is great enough to make abortion the dominate issue. And all of this happens without any claim of equivalence between the severity of abortion and any other issues.