Well, there could well be a liberal Catholic argument with it

As Finnis, Boyle, and Grisez make clear, their critique of the morality of forming even a conditional intention to kill non-combatants (whether the weapons one is contemplating using are nuclear or conventional) depends entirely on the rejection of the putative principle of moral judgment that directs the choosing subject to select that option which overall and in the long run promises to yield the net best proportion of benefit to harm.  This is the master principle of "proportionalist" ethics of the sort famously championed by liberal Catholic moral theologians such as Bernard Haring, Joseph Fuchs, Charles Curran, and Richard McCormick.  They and other liberal Catholic theologians who adopted the proportionalist method invoked this principle in defending contraception, homosexual conduct, direct abortion in some cases, and certain other practices that Catholic moral teaching condemns as intrinsically immoral.  Proportionalism seems incompatible with the belief that any specific moral norm can be an "absolute," that is, a norm that rules out certain forms of conduct always and everywhere, irrespective of the consequences of honoring it.  On a proportionalist understanding, there is no reason in principle to rule out in advance the possibility that forming a conditional intention to kill noncombatants in a retaliatory attack would, all things considered, probably produce the net best proportion of benefit to harm overall and in the long run.  Of course, for their own reasons, liberal Catholics might like Obama's policy of "narrow[ing] the conditions under which the United States would use nuclear weapons."  But, to the extent that they embrace a proportionalist understanding of fundamental ethics and moral theology, they are in no posistion to draw support from the work of Finnis, Boyle, and Grisez on direct killing and conditional intentions, or to claim that definitive Catholic teaching absolutely rules out the direct killing of noncombatants and the forming of even conditional intentions to kill innocent human beings.  They are scarcely in a posistion to criticize "Republicans" or others who oppose Obama's nuclear weapons policies on the ground that sound policy treats non-combatant immunity as a moral absolute.  No one will be surprised that my advice to them is to abandon proportionalism.  Then they could, with integrity, condemn the killing of noncombatants as a matter of strict principle, and call out Republicans and others (including most Democrats) who favor a policy that depends on the formation of conditional intentions to kill noncombatants all they want.  Of course, I would then invite them to rethink their views on sexual ethics, abortion, and some other important moral issues on which some liberal Catholics tend to be closer to the views of the New York Times editorial board than to the firm and constant teaching of the Catholic Church.