PHILADELPHIA, SEPT. 10 -- Former Democratic presidential front-runner Gary Hart, seeking to make his views a factor in the 1988 election debate, today called for a more creative and less confrontational U.S. foreign policy to respond to changes in the Soviet Union that he contends could make it "a more relevant superpower than ours in the 21st century."

In a speech to the Philadelphia World Affairs Council, Hart warned that if the United States continues down its "blind alley of excessive nuclear armament" it might wind up "militarily musclebound, uncompetitive . . . and broke." He said that Soviet leader Mikhail Gorbachev might prove to be "more far-sighted and insightful than we are."

Hart said Gorbachev's attempts to modernize the Soviet Union's economy could "redefine the superpower relationship" and that the United States must respond by "energetically seeking deep reductions in existing {nuclear} arsenals" and "more cooperative ventures in our mutual self-interest," including space exploration and an American-Soviet youth corps in the Third World.

Hart's speech was delivered two days after he announced that he will not attempt to become a presiden-

tial candidate again after withdrawing in May because of reports of extramarital relationships. "I am going to give speeches, and I am going to have an impact," Hart said.

Tonight's speech was the first of about two dozen on a range of issues he has scheduled this fall and winter before various business, university and issue-oriented groups around the country.

In it he said that the "traditional -- and increasingly dangerous -- militarization of our policy toward the Soviet government" needs to be replaced by "new political thinking" that will help prevent nuclear war.

"First, we must redefine the nature and terms of arms control . . . away from traditional efforts to limit numbers of weapons in various categories to the prevention of the use of nuclear weapons -- by anyone for any reason," Hart contended.

Prevention of the use of nuclear weapons must include a comprehensive test ban agreement, a "massive Manhattan Project-type joint venture to improve verification," eventual elimination of vulnerable first-strike systems and joint crisis-management centers, he said.

Hart also called for substantial joint reductions in conventional weapons and "cooperative ventures in our mutual self-interest" that would include cooperation in biomedical research, nuclear reactor safety and space exploration . . . . "

"Or let us even dream a little. Why not an American-Soviet youth corps in the Third World?" Hart continued. "Young teachers, doctors, agronomists and engineers who would teach, heal, cultivate and build instead of establishing military bases and weapons that kill."

He said it is time to "be testing Mr. Gorbachev instead of testing weapons." He described Gorbachev as "a modern man" who represents "the possibility of historic change in the Soviet Union."