THE REAGAN ADMINISTRATION's assault on the 20th century continued last week, and its target was a big one: the federal government's traditional commitment to defending civil rights. Reagan supported legislation approved by the Senate would severely limit the judiciary's authority to prescribe busing as a cure for school segregation.

Especially scary about that proposal is its implicit attack on another target: the Supreme Court. Last week's legislation--and a host of other bills pending before Congress many sponsored by Sen. Jesse Helms (R.N.C.)--would dramatically curtail the Court in dealing with social issues. If those efforts succeed, the forces of the New Right would be given free reign to roll back many of the civil liberties gains of the last 20 years, among them Court decisions legalizing abortion, limiting school prayer and setting forth stringent requirements for the application of the death penalty.

Last week's legislation represented the most stringent anti-bushing policies ever to win the approval of a House of Congress. Put forth by Helms and Sen. J. Bennett Johnson (D-La.), the rider to a justice Department allocation bill would forbid all courts from ordering busing of students living more than five miles or 15 minutes from a school. It would also apply retroactively to nullify much court-ordered busing around the nation.

In states like Maryland and Virginia, where busing has been widespread, officials predict the legislation could promote re-segregation. On the local front, if expected challenges to the constitutionality of the legislation fall short, the impact of Helms' initiative could be even more stark. More than half of the 35,000 Boston students who are currently bused to school ride more than 15 minutes--raising the spectre that Boston would have to retrench dramatically on its widespread desegregation plans.

Busing plainly has proven an imperfect answer to much school segregation. Particularly in the Northern cities in which it became widespread in the mid-1970s, its impact has been mixed, and is has frequently stirred up new racial tensions. But in some cities--particularly those of the Deep South, where it helped break up century-old segregation--it has had impressive effects.

At the very least, busing should remain a potential remedy for judges, as advised by local school boards and officials. Certainly Congress can recommend to the courts that they use busing only as a last resort in desegregation cases. But as legal analysts like Archibald Cox, Loeb Professor of Law, have frequently pointed out of late, congressional infringement on the powers of the courts sets a dangerous--and quite possibly unconstitutional-precedent. We urge Congress to avoid it--and to reject Helms's please to hamstring the third branch of government.

The Burger Court in recent years has retrenched somewhat on the gains made by its predecessor, the activist and rights-conscious tribunal of Earl Warren. Right-wingers have assailed the Court for not scrapping even more of that Court's "Liberal solutions. "But doubts about specific Court decisions are no excuse for limiting its authority as the branch most committed to defending civil rights. If the Reagan Administration continues its attempts to roll back the social legislation of the sixties, we may need a truly activist Court--one committed to defending civil rights and equipped to do so--more than ever.