This post is going to be full of sociological claims about which I am uncertain. It's okay if you disagree with me about them, so long as you do so nicely.
First claim: Thirty-five years ago, most conservatives were both very strongly in favor of the death penalty and strongly opposed to torture, where "torture" would have included waterboarding, if they had been asked about it.
Second claim: This is no longer true today. Now, conservatives who are strongly in favor of the death penalty tend to be the same ones who support at least some forms of torture, and conservatives who are opposed to those same forms of torture tend to be, at least, uneasy about the death penalty rather than strongly in favor of it.
Suppose these are both true. What caused the change?
The short answer is that I don't actually know, but I have a few conjectural causes, which are not mutually exclusive.
First conjectural cause: The increasing opposition of the Catholic Church to the death penalty, as represented by the statements on that subject in Evangelium Vitae, has made many conservative Catholics who were previously strongly pro-death penalty feel that they have to modify their views. In particular, it probably (and understandably) seems to many of them that in light of the current teachings they cannot, in response to the question, "Why is it morally right to execute X?" give the simple, old-fashioned response, "Because X has been duly convicted of a crime worthy of death."
Second conjectural cause: The previous support for the death penalty among conservatives had been made explicit long ago, because leftists have been attacking the death penalty for a long time. (Indeed, for a while, it was blocked by the U.S. Supreme Court.) But the previous opposition among conservatives to torture was largely implicit. In fact, I'm having to infer it implicitly. The conservative on the street usually didn't discuss the licitness of torture, even if he discussed a lot of political and moral things, because everybody believed that "we" (the U.S.) didn't engage in anything remotely like torture. Since the position was largely implicit, it was more vulnerable to change than an explicitly articulated and stubbornly held position. That change came about when it turned out that we have waterboarded people, etc., and the left flipped out. Since conservatives know that we cannot trust the left to set our policy regarding bad guys (and boy, do we know that!), there was a somewhat understandable reaction against the left, an assumption that if the left thinks something is wrong or too harsh, it probably isn't. Inductively, this is justified, since the left would like all foreign-captured terrorists to be treated with the same kid gloves they want used on domestic criminals, so that each terrorist can be the center of his own little grievance theater drama, complete with solicitous lawyers, contrived excuses for overturning perfectly legitimate convictions, endless and costly appeals, and irrational protestors who simultaneously claim that he is innocent and that what he did was justified.
Of course, we aren't just dealing with an inductive case. It's not like torture is a black box and we are being asked to pronounce on some undescribed action vis a vis evil enemies of the U.S. about which we know only that the left opposes it. But it's still not entirely surprising that the reaction happened, leaving us with people who, if they were around thirty-five years ago, would have been horrified to think of the U.S. waterboarding prisoners but who now support it.
That is my conjecture as to what happened. And it's a shame. There is something to my mind bracing and healthy about a person who recognizes, sternly, uncompromisingly, and forcefully, the importance of the death penalty, who takes a manly position also on self-defense, who is nowhere close to pacifism, but who views torture with unmitigated disgust and rejection. Can anyone imagine John Wayne torturing anyone? But I can certainly imagine him solemnly sentencing a murderer to death. And what about the Marine hymn? "First to fight for right and freedom, and to keep our honor clean."
My sociology may be all wrong. Maybe the old consensus existed only in my imagination. But I miss it, anyway.