COPENHAGEN, DEC. 12 -- The Reagan administration has abandoned its effort to persuade the Soviet Union and Congress to adopt the "broad interpretation" of the 1972 Anti-Ballistic Missile {ABM} Treaty, but will press ahead anyway with antimissile tests on a case-by-case basis, Secretary of State George P. Shultz said today.

Talking to reporters on a flight from Brussels to Copenhagen, Shultz made clear that the change was a tactical one that would not interfere with the administration's Strategic Defense Initiative, commonly called "Star Wars."

"We're trying to get away from the matter of narrow interpretation and broad interpretation and so on," Shultz said.

Instead, he said, the administration will seek congressional approval of specific tests without spelling out whether it is under the narrow or broad interpretation. If the Soviets object to a particular test, he said, it is up to them to decide how to react.

The administration announced more than two years ago that the ABM treaty's ban on testing missile defense systems did not apply to programs such as Star Wars that are based on technologies not in use at the time the pact was signed. At that time, officials said, the administration's program could be tested under the broad interpretation but not under the more traditional narrow one.

The reinterpretation drew complaints from Soviet negotiators, member governments of the North Atlantic Treaty Organization and members of Congress. Under a congressional budget restriction, the administration is banned from conducting any tests under the broad interpretation before Sept. 30, 1988.

In effect, the administration has now decided that the argument over semantics has been lost. However, Shultz made clear, the administration will continue to pursue the program as it sees fit.

A senior U.S. official said the administration was prepared to negotiate with Congress over specific test programs but would not attempt to establish agreed testing guidelines with Moscow.