Calling for new American leadership in world affairs, Sen. Edward M. Kennedy (D-Mass.) proposed yesterday that the United States call for an Indochina peace conference and consider granting most-favored-nation trading status to China and the Soviet Union.

In a speech to the Council on Foreign Relations in New York, Kennedy rejected the idea that American power in the world is declining. "The challenge is how to apply our strength effectively against the shifting sands of revolutionary change in individual nations and of continuing great power competition," Kennedy said.

In Indochina, he said, the objective of U.S. policy should be to give the Hanoi Government new inducements to reduce its dependence on the Soviets and cooperate with its neighbors to stabilize Southeast Asia.

An Indochina peace conference. Kennedy said, should be proposed by the United States, Japan and members of the Association of Southeast Asian Nations (ASEAN), first to try to arrange a cease-fire in Cambodia and to deal with the problems of Indochinese refugees.

Later, Kennedy said, the conference could consider broder issues of epace and stability in the region. In cojunction with progress toward these objectives, he recommended that the United States and Japan could offer broader economic cooperation with Vietnam, and the United States could normalize diplomatic relations with Hanoi.

"Had we moved to normalize relations Hanoi.

"Had we moved to normalize relations [with vietnam] last year," Kennedy declared, "it is at least possible that vietnam would not have concluded its new alliance with Soviet Union and would have limited its objectives in Cambodia, as China did [in its recent invasion] in Vietnam."

Kennedy deplored what he called "the present downward spiral of U.S.-Soviet relations," and urged that action be taken to reinvigorate detente. The granting of most-favored-nation tariff status and Export-Import Bank credits to the Soviets "can introduce major new incentives for a more positive relationship" between Washington and Moscow, Kennedy said.

He proposed granting these trade benefits to China at the same time by giving both countries a one-year waiver to the terms of the Jackson-Vanik Amendment, which ties such benefits to socialist countries to their emigration policies.

Kennedy said such moves should be postponed, however, until a new strategic Arms Limitation Treaty (SALT) has been signed and ratified, and that only most-favored-nation status be granted at firs. Credits could follow "if and when our relationships improve," Kennedy said.

To grant trade concessions to China but not to the Soviet Union would be a mistake, Kennedy said, because it would aggravate Soviet-American relations while suggesting the United States thought its dealings with China, "a regional power with limited strength," were more important than relations with the Woviet Union," a global power with far greater strength."