The Supreme Court was tidying up after itself last week. It sort of shrugged and muttered, "Well, on the other hand . . ."

It said that a ruling it rendered in 1976 (with eight of today's nine justices involved) was, well, err . . . The word five justices tried to avoid is "incoherent," so let us just say: mistaken. The result was a reversal that scrambles the ideological eggs nicely, and gives this court much-needed practice saying "Oops! Sorry about that."

The case concerned a dry-as-dust matter: must the city of San Antonio pay transit workers in accordance with federal labor law? The court, divided 5-4, said yes, the city must conform to Congress. But in 1976 the court reached an opposite conclusion, 5-4, in a similar case. It ruled that the Tenth Amendment prevents Congress from legislating where states are exercising "traditional governmental functions."

The adjective "traditional" was an evasion masquerading as a standard. It served an ideology that can be called Jeffersonianism, an ideology of nostalgia, abstraction and almost charming disregard for modern history. It favors the kind of light, decentralized government suitable for the kind of uncomplicated, unindustrial society Jefferson favored.

The adjective "traditional" also was an example of one of this court's two specialities: it was a muzzy criterion, and hence was an invitation to additional, "clarifying" cases. (The court's other speciality is loud complaint about its work load.)

The 1976 decision rested, precariously, on the thundering anachronism of the Tenth Amendment's assertion that, "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the states respectively, or to the people." The opinion was written by Justice William Rehnquist. He is, deservedly, a pinup of conservatives because he generally favors judicial restraint, understood as deference to the elected, political institutions of government.

Conservatives regarded the 1976 ruling as a sign that the court was taking up where it had left off 40 years earlier, when it had stopped putting sand in the gears of the New Deal. In the latter half of the 1930s, the court became more deferential to Congress as it concentrated power in the central government. The concentration was facilitated by the court's expansive construction of the Constitution's commerce clause, which vests in Congress the power to regulate interstate commerce.

The 1985 reversal of the "conservative" victory of 1976 occurred because Justice Harry Blackmun, no conservative, changed sides, returning to a more permissive interpretation of Congress' powers under the commerce clause. He joined the 1976 dissenters, including the three other most liberal justices -- Brennan, Marshall and Stevens -- and the generally more conservative Byron White. So now the court has said that the "traditional function" criterion for assigning states' rights is not only unworkable but is inconsistent with the way American government has evolved for 50 years.

Furthermore, the court has now said: Boundary-marking by judicial fiat is not the proper way to establish the federal- state balance. Courts should not concoct limits on Congress' power by boldly asserting a definition of a state's sovereignty. Courts should step back and allow the political process to make most of the adjustments in the balance of federal and states' rights.

Before conservatives lower their flags to half mast, they should note that the Reagan administration entered a brief on behalf of the position opposed by the four conservative justices (Burger, Powell, O'Connor,Rehnquist) and for the position ardently favored by organized labor and adopted by Blackmun. Furthermore, in 1976 Robert Bork, who today is one of the nation's finest conservative judges, was U.S. solicitor general. He argued for the position that lost then but has now prevailed.

The 1985 decision limits states' rights, so it should offend conservatives, right? Not so fast. Life is so cruel that even conservatives have to choose between competing values. In this case they could not have "states' rights" and "judicial restraint" too. It is Blackmun's opinion that is deferential toward the political process and modest, not to say chastened, about the proper role for judicial power and the limited ability of judges to fine- tune society's institutions.

Bashing the federal government is the preferred indoor sport for some conservatives, but correct thinkers among conservatives are not sentimentalists, least of all where the Tenth Amendment is concerned. They know that an expansive notion of states' rights is a Jeffersonian residue. They know it is unsuited to this brawny, complex nation, a nation that fits the vision of Jefferson's rival, the correct thinkers' hero -- Alexander Hamilton.