Without a definitive judicial ruling or other galvanizing event, a perennial American argument is ending. Capital punishment is withering away.

It is difficult to imagine moral reasoning that would support the conclusion that an injustice will be done when, years hence, the death penalty finally is administered to Dzhokhar Tsarnaev, the Boston Marathon terrorist who placed a bomb in a crowd and then strolled to safety. Sentencing to death those who commit heinous crimes satisfies a sense of moral proportionality. This is, however, purchased with disproportionate social costs, as Nebraska seems to be concluding.

Nebraska is not a nest of liberals. Yet on Wednesday its 49-member unicameral Legislature passed a bill abolishing the death penalty 32-15. Gov. Pete Ricketts, a Republican, vows to veto it.

This comes at a time when, nationwide, exonerations of condemned prisoners and botched executions are dismayingly frequent. Nebraska’s death penalty opponents, including a majority of Nebraskans, say it is expensive without demonstrably enhancing public safety or being a solace to families of murder victims. Some Nebraska families have testified that the extended legal processes surrounding the death penalty prolong their suffering. That sentiment is shared by Bill and Denise Richard, whose 8-year-old son was killed by Tsarnaev.

Last month, the U.S. Supreme Court heard oral arguments about whether one component of a three-drug mixture used in lethal injection executions — and recently used in some grotesquely protracted ones — is unreliable in preventing suffering that violates the Eighth Amendment proscription of “cruel and unusual punishments.” States use the drug in question because more effective drugs are hard to acquire, partly because death penalty opponents are pressuring drug companies not to supply it.

For this, Justice Antonin Scalia blamed a death penalty “abolitionist movement.” Justice Samuel Alito asked, “Is it appropriate for the judiciary to countenance what amounts to a guerrilla war against the death penalty, which consists of efforts to make it impossible for the states to obtain drugs that could be used to carry out capital punishment with little, if any, pain?” Justice Anthony Kennedy wondered, “What bearing, if any, should be put on the fact that there is a method, but that it’s not available because of opposition to the death penalty? What relevance does that have?”

The answers are: Public agitation against capital punishment is not relevant to judicial reasoning. And it is not the judiciary’s business to worry that a ruling might seem to “countenance” this or that social advocacy.

The conservative case against capital punishment, which 32 states have, is threefold. First, the power to inflict death cloaks government with a majesty and pretense of infallibility discordant with conservatism. Second, when capital punishment is inflicted, it cannot later be corrected because of new evidence, so a capital punishment regime must be administered with extraordinary competence. It is, however, a government program. Since 1973, more than 140 people sentenced to death have been acquitted of their crimes (sometimes by DNA evidence), had the charges against them dismissed by prosecutors, or been pardoned based on evidence of innocence. For an unsparing immersion in the workings of the governmental machinery of death, read “Just Mercy” by Bryan Stevenson, executive director and founder of the Equal Justice Initiative.

Third, administration of death sentences is so sporadic and protracted that their power to deter is attenuated. And the expensive, because labyrinthine, legal protocols with which the judiciary has enveloped capital punishment are here to stay. Granted, capital punishment could deter: If overdue library books were punishable by death, none would be overdue. But many crimes for which death is reserved, including Tsarnaev’s crime of ideological premeditation, are especially difficult to deter.

Those who favor capital punishment because of its supposed deterrent effect do not favor strengthening that effect by restoring the practice of public executions. There has not been one in America since 1937 (a hanging in Galena, Missouri) because society has decided that state-inflicted deaths, far from being wholesomely didactic spectacles, are coarsening and revolting.

Revulsion is not an argument, but it is evidence of what former Chief Justice Earl Warren called society’s “evolving standards of decency.” In the essay “Reflections on the Guillotine,” Albert Camus wrote, “The man who enjoys his coffee while reading that justice has been done would spit it out at the least detail.” Capital punishment, say proponents, serves social catharsis. But administering it behind prison walls indicates a healthy squeamishness that should herald abolition.

© 2015, Washington Post Writers Group