In
Don't Think of an Elephant, Lakoff discusses the apparent philosophical inconsistency of opposing war and capital punishment, on the one hand, and supporting abortion rights and euthanasia, on the other. He resolves the inconsistency by deriving the left and right positions on these issues from underlying metaphors of family structure. But there's a simpler, more persuasive element cutting across all these issues:
Government intervention.
When there's a question of whether or not a particular person/fetus should live or die, the consistent liberal position is that the government shouldn't involve itself. Rather than (or in addition to) arguing the individual merits of capital punishment, abortion rights, euthanasia, etc., we should be pushing this point. (Ironically, it's an argument that ought to particularly appeal to small-government conservatives.)