The Supreme Court yesterday sharply limited the power of a state to expand the protections against oil spills provided by the federal government.

Over the protests of 16 coastal and Great Lakes states including Maryland and Virginia, the justices ruled 6 to 3 that a provision of the Constitution making federal law "the supreme law of the land" prevented the state of Washington from:

Barring from Puget Sound, an estuary consisting of 2,500 square miles of inlets, bays and channels, supertankers with a cargo capacity of 125,000 or more deadweight tons (DWT).

Requiring tankens of between 40,000 and 125,000 DWT to have certain "safety features," such as twin screws, double bottoms under all oil and liquid-cargo compartments, and two operating radars, one of them for collision avoidance.

By a vote of 7 to 2, however, the court said Washington was free to require tug escorts for tankers of at least 40,000 DWT that don't have the safety features.

And the justices held unaminously that the state also could require pilots on tankers of at least 50,000 DWT - provided they are engaged in foreign trade.

The court took these actions in ruling on whether the Washington Tanker Law of 1975 was pre-empted by the federal Ports and Waterways Safety Act of 1972.

Justice Byron R. White, in the opinion for the court, wrote that a panel of three federal judges had gone too far in invalidating the state law in its entirety. Only three other justices joined White in the full opinion.

The case arose on the day the state law became effective, with the filing of a challenge to its constitutionality by Atlantic Richfield Co. (Arco), which operates a refinery in the northern part of the sound, and later by Seatrain Lines, which is building four 225,000-ton supertankes.

Of 105 deliveries of crude oil to the refinery between 1972 and 1975, 15 were in tankers of at least 125,000 DWT, and 80 in tankers of 40,000 to 125,000 DWT.

The record of the case shows that no tanker now afloat has all of the safety features required by the state law. The tug-escort requirement will cost ARCO an estimated $277,500 annually, but the requirement may be short-lived because the federal government is considering pre-emptive regulations.

The court took other actions: ABORTION

In its 1973 abortion decision, the court held that up to the 24th week of gestation, the fetus is presumed to lack the capability of meaningful life outside the womb, and that the decision whether to abort therefore must be the mother's alone. Starting with the 24th week, however, the fetus is presumed to have a capability for healthy survival; the state therefore can prohibit abortion at that point.

In 1974, Pennsylvania passed a law requiring that before peforming an abortion, a physician must determine whether the fetus "may be viable" (able to survive outside the womb, even if with artificial aid), and, if it may be, the physician must use the care necessary to protect the fetus' life, so long as he does not thereby endanger the mother's life or health.

In a trial before a panel of three federal judges, the state described the law as "simply a codification" of the 1973 Supreme Court ruling.

But the Obstetrical Society of Philadelphia objected that the "may be viable" period runs from 20 to 26 weeks; that the law conseequently allows the state illegally to regulate abortions before the 24th week; that the criminal penalties facing a doctor "chill" a woman's constitutional right to an abortion before 24 weeks, and that the laws impairs the ability of couples who may be carriers of genetic disorders such as Downs syndrome to have, and to act upon, the tests that will tell whether the fetus is afflicted.

The panel of judges ruled the law unconstitutional. The Supreme Court will review the ruling. VOTING RIGHTS

The court held 6 to 3 that the Voting Rights Act of 1965, which was intended to end racial discrimination at the polls, applies to Sheffield, Ala., because it is a political subdivision of a state targeted by the law.

In another Alabama case, the court agreed to decide whether a municipality may govern residents of an adjoining geographical area while barring them from voting in its elections or participating in its government. EMPLOYMENT BIAS

In a case from Tennessee, the court, for the second time in nine months, let stand a ruling that a white deprived of interracial associations by unlawful discrimination by an employer is empowered to seek government intervention to end the discrimination.