"When a Catholic does not share a candidate's stand in favour of abortion and/or euthanasia, but votes for that candidate for other reasons, it is considered remote material cooperation, which can be permitted in the presence of proportionate reasons."
The question is, what would such proportionate reasons look like?
And since the purpose of Ratzinger's memo is to explicitly highlight the relative gravity of abortion/euthanasia vis-a-vis other concerns, the burden seems to be squarely on the shoulders of those who would propose that any other concerns -- those, I mean, which are 'on the table' in the upcoming American elections -- are genuinely equal to the moral gravity of these fundamental matters, literally, of life and death.
My understanding is that the reasons whose presence permit remote material cooperation must be proportionate, not to the evil act itself, but to the degree of cooperation involved. If that's the case, there need not be "on the table" concerns equal to the moral gravity of abortion to vote for a pro-abortion candidate.
So, taking an example Jamie mentions, to licitly use a product made by a company that donates money to Planned Parenthood, you don't need a reason proportionate to the abortions Planned Parenthood performs, but one merely proportionate to the level of cooperation in donating money to Planned Parenthood that using the product constitutes.
This raises the question, what level of cooperation in legal abortions does voting for a pro-abortion candidate constitute? The answer is, "It depends."
Some people think the answer is, "The level of cooperation is miniscule, at least for any candidate I want to vote for." The problem with this, it seems to me, is that voting for a candidate constitutes equal cooperation with every policy a candidate holds. Yes, if you vote for someone because of a policy, you are formally cooperating with that policy, while if you vote despite a policy, you are only materially cooperating with it. But the effect of your vote is the same either way. If the remoteness of cooperation with unfavored policies is the same as the remoteness of cooperation with favored policies, in evaluating them for proportionality you must consider the policies in themselves.
The Church has been absolutely clear that human life issues are the most important political issues of our time. (And, for that matter, that there is a difference between "human life issues" and "quality of life issues," despite the efforts of many politically liberal Catholics to fuse them.)
Yet I think the Church has also been clear that human life issues exist on a continuum, that though they are far more important than all other issues, they are comparable. If so, then it is possible for the difference on human life issues between candidates to be less than the difference on other issues, in which case proportionate reasons for voting for a pro-abortion candidate would exist.