Twenty years ago, a much more rapid advance seemed likely. Some sort of right to suicide seemed like a potential extension of “the right to define one’s own concept of existence” that the Supreme Court had invoked while upholding a woman’s constitutional right to abortion. Polls in the 1990s consistently showed more support — majority support, depending on the framing — for physician-assisted suicide than for what then seemed like the eccentric cause of same-sex marriage.

Yet the latter cause has triumphed sweepingly, while voluntary euthanasia has advanced only haltingly. Part of the explanation lies with the Supreme Court, which in 1997 ruled 9 to 0 that the Constitution does not include a right to suicide. But the court would not have ruled as it did absent a deeper reality: Many liberals seem considerably more uncomfortable with the idea of physician-assisted suicide than with other causes, from abortion to homosexuality, where claims about personal autonomy and liberty are at stake.

Conservatives oppose assisted suicide more fiercely, but it’s a persistent left-of-center discomfort, even among the most secular liberals, that’s really held the idea at bay. Indeed, on this issue you can find many liberal writers who sound like, well, social conservatives — who warn of the danger of a lives-not-worth-living mentality, acknowledge the ease with which ethical and legal slopes can slip, recognize the limits of “consent” alone as a standard for moral judgment.