It is the liberal nightmare come true. In the twilight of the Reagan era, conservatives finally get a secure lock on the Supreme Court and begin methodically overturning all the liberal decisions of the postwar era. It certainly looked like the beginning of a judicial auto-da-fe' April 25. Lawyers for both sides in Patterson v. McLean Credit Union thought they were arguing about a minor point of interpretation of a 12-year-old civil rights ruling. Out of the blue, by a 5-4 vote, the court asked them to discuss whether the whole ruling shouldn't simply be reversed.

This is a nightmare liberals have managed to stave off for almost two decades, ever since Richard Nixon had two court openings at the beginning of his term. Even before President Reagan's latest appointment of Anthony Kennedy, seven of the nine justices had been chosen by Republican presidents. Yet until now, the miraculous unreliability of Supreme Court appointees has prevented a ruthless conservative majority from emerging.

Liberals thought they'd pulled the fat from the fire once again last summer when they kept Robert Bork off the court. Conservatives are enjoying the irony: Bork, as solicitor general, successfully argued the pro-civil-rights side of the case Justice Kennedy has now voted to consider overturning. If Bork had been on the court, and if he believed what he said in 1976, this would not have happened.

On the other hand, conservatives have their own, more sophisticated, nightmare. Since the issue in this case is an 1866 statute, not the Constitution, Congress is free to overturn any court ruling. This year, in the Grove City bill, Congress reversed a Supreme Court decision narrowly interpreting part of the 1964 Civil Rights Act -- in the process delivering a poke in the eye to President Reagan.

Former Reagan administration official Michael Horowitz argues that exploitation of a 5-4 majority may backfire on conservatives -- just as, he says, excessive use of judicial power in the past backfired on liberals. "But for the abortion decision . . . Ronald Reagan {might not} have been . . . elected president. Thus, if we conservatives win our policy victories through fifth votes . . . our con-duct will galvanize powerful counterforces."

Thinking this business through is tricky for liberals as well, since the 1976 decision now at issue was pretty clearly wrong. A post-Civil War law says that "all persons . . . shall have the same right in every State and Territory to make and enforce contracts . . . as is enjoyed by white citizens." For a century, this was assumed to mean simply that the government couldn't have different commercial rules for black and white people. The interpretation the court accepted in 1976 is that this law forbids racial discrimination by private individuals.

This ruling is tremendously useful to civil rights lawyers, since it fills some holes in modern civil rights legislation. The 1964 Civil Rights Act, for example, excludes employers of fewer than 15 people, has a short time limit on filing complaints and sharply restricts damage awards. However, the very usefulness of the ancient 1866 statute, and others like it, shows the implausibility of their current interpretation. The 1964 Civil Rights Act emerged from a lengthy national debate. Was that exercise unnecessary because an even broader law was already on the books? Martin Luther King Jr. and George Wallace, Hubert Humphrey and Strom Thurmond, would have been surprised to hear it.

Dissenting from the April 25 decision, Justice John Paul Stevens wrote that it was unfair "to recognize an equality right . . . and then to declare unceremoniously that perhaps we were wrong and had better reconsider our prior judgment." But Stevens thought the court was wrong in 1976 as well. "For me," he wrote back then, "the problem . . . is whether to follow a line of authority which I firmly believe to have been incorrectly decided." He supported the "incorrect" interpretation because it "is now an important part of the fabric of our law."

With so many liberal doctrines now woven into "the fabric of our law," liberals like the idea that they should be preserved even if most justices think they are wrong. Conservatives complain that this turns the Supreme Court into a one-way legal ratchet: liberal majorities add doctrines but conservative majorities can't subtract them. How much respect the court should give its own past rulings is a perennial question that will be paramount in the coming years. Certainly, though, it represents an extreme approach for the court to announce it has decided to reopen an old controversy when nobody has even asked it to do so.

This week's (unsigned) majority opinion scolded the dissenters for suggesting that civil rights laws deserve special respect. It noted pompously that each justice takes an oath to "do equal right to the poor and to the rich." Since the majority's specific concern is that the rights of the poor have been interpreted too broadly, this remark recalls Anatole France's crack about the law in its majesty forbidding rich and poor alike to sleep under bridges. The court's conservative majority may be correct in this case, but the relish with which they take it up is unnerving.