NASHUA, N.H., MARCH 23 -- Sen. Joseph R. Biden Jr. (D-Del.) said here today that he believes the Reagan administration's real goal in seeking a broad interpretation of the 1972 Antiballistic Missile (ABM) Treaty is to achieve nuclear superiority over the Soviet Union.

"I believe what's at stake here -- and I'm not even sure if the president is fully signed on -- is a clear determination . . . to believe that we can in fact seek and gain nuclear superiority in a way that will enable us to somehow change the way the Soviets do business," said Biden, who is expected to formally announce his bid for his party's presidential nomination this spring.

Biden has long been critical of the administration's Strategic Defense Initiative (SDI), and more recently of the attempt to reinterpret the ABM treaty to allow earlier development of that program. He has argued previously that a functioning defensive system would upset the nuclear balance by increasing the Soviet sense of vulnerability to a first strike.

In his speech here, Biden noted that the administration already has the option to unilaterally abrogate the ABM Treaty -- but is "unwilling" to do so because that would mean having to "explain to the American people" its ultimate designs.

Biden said that if the administration rejects an interpretation that three successive presidents have accepted, "it means that every treaty every future president attempts to negotiate will find itself in real trouble."