At this point, it becomes a prudential decision—it has to.
Many who are very strident on this issue apply this very reasoning, and conclude they will vote for McCain over Obama. I will not condemn them, but note: they are, thereby, materially cooperating with the evil McCain endorses.
Ironically, some of the same people are arguing, absolutely, one may never, ever, under any circumstances, vote for a candidate who embraces something that is gravely evil (such as abortion)—they make that argument in relation to Obama, but then abandon that argument in relation to McCain.
After all, if you took that approach, you could not vote for Bush—because many fail to note that while Bush took a position against most abortions, he was in favor of allowing some of them—for rape and incest. While being against most is very good, being for any is gravely wrong.
But many prolifers decided that while they could not formally cooperate with Bush in this regard, they saw materially cooperating with him—by supporting him—to be justified as a way to prevent a worse evil—the election of someone who embraced abortion-on-demand. And they further reasoned that, under the circumstances, their cooperation with Bush in this matter was more remote, since we’re not faced with the immediate prospect of outlawing all abortions, with Bush the obstacle to saving those additional lives.
That’s acceptable reasoning, but again, it is prudential reasoning that justifies voting for a candidate who nonetheless is wrong on a non-negotiable. And it’s the same reasoning applied to voting for McCain, despite his support for baby-destroying research (which Bush supports, remember? He came out in favor of our tax dollars being used for it, with restrictions).
But given all that, this is why you cannot say, as some do: "you cannot, under any circumstances vote for a candidate who endorses ____ (insert a non-negotiable)."
The actual answer is, there are some circumstances where you may; but what you may not do is share the intention to do the evil—you may not support the candidate because of his or her embrace of something evil.
What are the circumstances? What does the Church say on this subject?
Rich Leonardi points to two sources for guidance—one, the observations of then-Cardinal Ratzinger (now Pope Benedict) in 2004, and the other, Bishop Nicholas DiMarzio of Brooklyn, who oversaw the drafting of "Faithful Citizenship" while chairman of the USCCB's domestic policy committee:
BROOKLYN, N.Y. (CNS) -- A "hierarchy of values" exists, which means not all political issues are of equal value, said Bishop Nicholas DiMarzio of Brooklyn. "Our faith must inform our political decisions," he said, and Catholic voters are obliged to distinguish "between moral evil," such as abortion, "and matters of prudential judgment," such as tuition tax credits. ... "In our own country, despite significant victories that extend protection to the unborn, this modern slaughter of the holy innocents continues because of the policies of unscrupulous politicians," he said.
"Only in circumstances that are extraordinarily hard to contemplate may a Catholic voter support a proponent of so great an intrinsic moral evil," the bishop said (Bolding of text added for emphasis).
I may be wrong, but I think the bishop’s comments here are directed to a situation in which the contrast between candidates is clear: one is for abortion, the other is against.
It seems to me the circumstances are not so "extraordinarily hard to contemplate": we might have faced a choice this year between Obama and former New York Mayor Rudy Giuliani, who is only marginally less pro-abortion than Obama. Again, McCain’s position on embryonic stem-cell research makes him, in DiMarzio’s words, "a proponent of so great an intrinsic moral evil"—but on a lesser scale than Obama. The circumstances in which one may feel bound to vote for that proponent—McCain—are to avoid a worse proponent—Obama. Again, not so hard to contemplate.
For my part, barring some consideration I haven’t thought of at this point, I shall not vote for either. But it is precisely because of this sort of dilemma I cannot agree with the blanket statements many are making.