In arguing that "the position that it is fine to vote for the Republican candidate as a means to limiting the potential harm of the Democratic candidate" is not established merely by Catholic moral principles, Zippy writes, first:
Whether or not there is a proportionate reason is a question of both principles and facts.
Then, a couple of paragraphs later:
Whether or not there is a proportionate reason to vote for a certain candidate is not ultimately a matter of opinion. It is ultimately a matter of fact. Opinions do have dignity, particularly in the face of ambiguous facts and complexity. But their dignity arises ultimately from the truth.
Zippy's primary concern, I think, is to get acknowledgement that facts have anything to do with evaluating proportionate reasons, that a person's reason isn't proportionate merely because the person genuinely considers it proportionate. But I'd say he overstates the case when he says the proportion is not ultimately a matter of opinion. It is a matter of both facts and opinions -- and principles, too, of course.
It's a matter of facts because things aren't the way we say they are merely because we say they are. If it is a fact that a candidate supports a policy of murdering the innocent, then the candidate supports a policy of murdering the innocent no matter how much I might want to obscure or downplay or deny it.
But proportion also depends on opinion, and not merely in the face of ambiguity or complexity.
Suppose I judge that a certain act of remote material cooperation with evil will produce a good that is proportionate to my level of cooperation. Then, since that's exactly what a proportionate reason is, in my judgment I have a proportionate reason.
Now, my judgment needs to be well-founded, but (except for trivial cases) it cannot be entirely founded on facts and principles. Something entirely founded on facts and principles isn't a matter of opinion, but knowledge, and a proposition about future contingent things cannot be a matter of knowledge (for humans at least).
As I wrote before, if I want to justify my claim that I have proportionate reasons to cooperate with evil, it's not enough to merely give the reasons; I need to be able to argue that my reasons really are proportionate. Like any argument, mine may be valid (meaning that if the facts are as I understand them then my reason really is proportionate) or sound (meaning that it's valid and the facts actually are as I understand them).
But even a sound argument for proportion will [almost always] need to rely on opinion about uncertain things, where the uncertainty comes not just because I don't know some things, but because some things don't yet and may never exist.