It was tactful of the Supreme Court to endorse the federal ''preventive-detention'' law at the expense of such unsavory characters as Tony Salerno and Vincent Cafro, accused members of a New York City ''crime family.''

But the reputation of the victims doesn't excuse the decision, which is arguably the grossest judicial assault on personal liberties since the court sanctioned the Japanese-American detentions of World War II.

It is Nixon's revenge -- a long-delayed but ironic triumph for our unindicted coconspirator president. Richard Nixon and his attorney general, John Mitchell, launched the crusade for preventive detention nearly 20 years ago. Congress soon yielded, but it has taken the court longer.

The majority of six includes some of the court's more thoughtful justices. It isn't surprising, then, that the chief justice's majority opinion adopts a defensive tone, suggesting a guilty sense that the court is on boggy ground.

George Orwell says in one of his great essays on language that insincerity is the enemy of clarity: disreputable propositions require cloudy words. That axiom is certainly borne out here.

For instance, the chief justice tells us that an American citizen locked up indeterminately without trial has ''liberty interests'' which must be balanced against society's competing ''interest in public safety.'' Of course a personal liberty, such as the Eighth Amendment guarantee against excessive bail, is less easily compromised than a mere ''liberty interest,'' which makes the Bill of Rights sound a bit like a joint-stock company.

Preventive detention, the chief justice further explains, is only ''regulatory,'' not penal. We know that, he declares, because that was the intent of Congress: ''The legislative history . . . indicates that Congress did not formulate the pretrial detention provisions as punishment.'' The implication is that Congress may disinfect its unconstitutional behavior by claiming constitutional motives. Preventive detention may look and feel punitive to those subjected to it; but it is merely ''regulatory.''

An even more sinister contortion of language appears further along. A criminally accused person denied bail on grounds of ''dangerousness'' is called, by the chief justice and the majority, a ''putative offender.'' This is someone who is accused of but not yet tried for a crime -- i.e., a person denied the customary presumption of innocence. The court has set its seal of approval on the presumption of guilt, but blushes to call it that.

But the decision's offense against fundamental rights is far more important than its offense against the English language.

It is as true today as when the late Sen. Sam Ervin Jr. was fighting earlier versions of ''preventive detention'' that the idea, aside from being constitutionally dubious, is both intellectually dishonest and impractical.

It is intellectually dishonest because it assumes judges can predict what this or that defendant might do on bail. When Ervin challenged that notion 15 years ago, the Nixon Justice Department sought from the Bureau of Standards a formal study of the predictability of criminal behavior. When the bureau reported ''no reliable prediction mechanism'' existed, the acting attorney general tried to suppress the report.

Nothing has changed. But the Rehnquist Court's response is that ''there is nothing inherently unattainable about a prediction of future criminal conduct.'' Right; there isn't. Science cannot say with finality that it is ''inherently'' unattainable; that would foreclose the future. What can be confidently said is what the Bureau of Standards said more than a decade ago -- that no such technique of prediction has yet been discovered.

As a practical matter, preventive detention wastes and disperses scarce judicial resources. The chief justice boasts twice that Congress has hedged preventive detention with many elaborate procedural safeguards.

But why clog our already logjammed courts with proceedings looking to merely temporary confinement? Why not devote the same energies to speeding up our laggardly prosecution of persons who are regarded as menaces to society?

The answer to the Salernos and Cafros and other not-very-nice people is not to sacrifice ancient liberties on their account. It is to bring them -- with every customary protection of their liberty -- to the swift and certain justice they so richly deserve.