The Supreme Court, stepping into a dispute that could determine the future of commercial nuclear power, agreed yesterday to review a case that tests whether states can ban construction of new reactors.

The case shapes up as a major confrontation between the nuclear industry and at least seven states that have placed moratoriums on new nuclear plants until the federal government develops a means of disposing permanently of high-level radioactive waste.

The justices next fall will hear an appeal filed by two California utility companies--backed by the Reagan administration--urging the high court to make an "authoritative determination" on whether a state interferes with federal authority by setting its own process for approving new nuclear facilities.

Other states imposing similar restrictions on nuclear power plant construction are Connecticut, Maine, Oregon, Montana, Maryland and Wisconsin.

In other action, the court:

* In a unanimous ruling, gave approval to a Medicaid policy used in 16 states that treats welfare recipients better than some low-income, retired and disabled workers in providing benefits under the state-federal medical assistance program. "Powerful equities unquestionably support the elderly and disabled claim of unfair treatment," Justice John Paul Stevens wrote for the court. But, he said, "A belief that an act of Congress may be inequitable or unwise is of course an insufficient basis on which to conclude that it is unconstitutional."

* Agreed to settle an unusual dispute involving the court itself: whether protests and passing out leaflets should be permitted on the grounds and sidewalks around the Supreme Court building. At issue is a federal appeals court decision striking down a law that allowed Supreme Court police to arrest anyone who tried to demonstrate or pass out written material around court building.

* Said it will consider whether police need a warrant to track a criminal suspect with a concealed "beeper" that transmits radio signals. The court will review a ruling throwing out a Minnesota drug case because police tracked the suspect by putting a beeper in a drum of chemicals.

* Ruled unanimously that federal judges should not interfere with state disciplinary proceedings against a lawyer. Writing for the court, Chief Justice Warren E. Burger noted that under a 1971 ruling, the high court established a "strong federal policy" against U.S. court interference with pending judicial proceedings, "absent extraordinary circumstances."

* Agreed to decide how federal officials must determine whether disability claims should be turned down because the people filing them are capable of some gainful employment.

Sparking the nuclear power case were 1976 amendments to California's Warren-Alquist Act, which imposed a moratorium on state certification of new reactor construction until a system is established to dispose of high-level nuclear plant waste.

The federal government has yet to create a method for storing such wastes, which can remain dangerously radioactive for many thousands of years.

The Energy Department's most recent schedule estimates the first permanent, high-level nuclear waste dump will start operating in the mid-1990s. Congress is considering legislation to create an underground disposal site.

The utilities involved in the case the court will hear next fall are Pacific Gas & Electric and Southern California Edison. They are being supported by the administration, which has strongly advocated greater reliance on nuclear power.